Just received my Global Roaming SIM (I'm in the US). Why is it prohibited from registering on either of AT&T's networks? I can register on only T-Mobile.

That is probably who they have roaming agreement with. When I used it in Nigeria last week, it would only register on 1 of the 5 networks there. It did work ok however.

The coverage list at http://www.globalroaming.mobi/pdf/Gl...20partners.pdf shows 310-410 (AT&T, formerly Cingular) as valid.

Also, is there a reason for charging by the minute for incoming calls even in Zone 1?

I think that price is based on the fact that they offer a free DID. However, I do not that they offer any kind of DID (free or rental) at the moment. So, that makes them less desirable compared to the other sims out there. For example, why would I use celtrek (or even maxroam) for incoming calls in the EU when travelsim, and similar sims, offer free incoming calls in most of those countries.

These calls to a Travelsim etc. calls are NOT free. In fact, calling an Estonia cell number from the U.S. is very expensive. Calling Isle of Mann numbers are cheaper - but of course = not free. Many users fail to take into accoiunt the charge that shows up on their phone back home that called the "free" incoming. To compare pricing, you need to add the cost incurred on both ends.
Outgoing calls are cheaper, it appears, than incoming in some cases.

...s

Very True. The calls are not free. The caller pays a premium for calling a mobile. However, in my comparison, my travelSim(Isle of man #) can be forwarded to or reached for around $0.20. The Base belgium number that both maxroam and celtrek use can be reached about $0.20 (higher in most cases). And we then add the cost to receive a call $ 0.20 or more in most countries. It's obvious that it will cost me at least double to use the Belgium based sim vs Isle of man one.

And my analogy above is probably the reason why Adam917 is asking why they are charging for incoming calls in these zones where other sims do not, and the caller still has to pay the premium to reach the number anyway.

Bingo. I certainly was thinking of what you just said when writing the original question, but forgot to mention it. I wonder when we'll start seeing global SIMs with numbers that aren't such a premium for callers to call. Of course IoM is better than Estonia but that's usually still not good enough for most Americans for instance (& I bet plenty of other people), that either want to dial a US number or the American end-user who would like to just forward all their calls to the global SIM & not incur extra charges from those incoming calls when travelling.

The capability should already exist to give the user a number from the country they would like to be reached at (in my case, I'd prefer any US number, I'm sure most Europeans would prefer a land-line number from their own country). From what I understand, getting SMS to work like this is more difficult, as the SIM can have only a very limited amount of numbers assigned to it, hence the former Celtrek SIMs having a separate voice number & SMS number.

Get my drift?

FREE incoming is certainly the reason that brought so many players out of business.
I do not see any rationality in FREE.
Funny how some expect us 'alternative operators' to offer FREE service while main carriers are stealing our $$ for last decade over excessive 'roaming' charges between same operator. For example Tmobile Germany/Tmobile USA/Tmobile UK. They do their own clearing. 100% profit on a 3Euro/min call.
I understand customer wants more but seriously you all have a job and certainly do not work for free. A free SIM + free shipping + free incoming + toll free number is what everybody wants but this won't happen. And if some fools wants to to try it, they will join the deadpool of companies that have tried.
My 2cts.
Sorry to jump in like this but sometimes obvious things must be said.

Best to all

Whoa whoa whoa there. I certainly don't mean free everything, but wouldn't it simply be logical to give free incoming at least in popular countries to turn customers away from reverting to buying a local SIM in each country they go? Would you rather have some business than none at all? I personally would be willing to pay a premium for outgoing if I knew my incoming will be covered. Heck, I would be willing to pay a separate perhaps annual fee to ensure it, that way you would be guaranteed a certain amount for each user and the user still gets what they want.

I take this POV for both calls and SMS (which from my understanding usually costs more to send in Europe vs the US, but in the US we pay for incoming SMS which in my eyes is very unfair, as spamming can end up costing users a lot of money). I wonder how the new roaming regulations in the EU will negatively affect both operators and customers.

You are confusing premium termination and regular EU mobile termination.
Base belgium is no premium number. If you call from EU you will be charged like a 'normal' EU mobile termination. There is no 'premium' to it. 1 min to a Base belgium number is exactly the same cost then 1min to a Vodafone UK or O2 ireland or SFR France.
Most people in EU have plans with Min within Europe. All calls to a Base Belgium number are included in those minutes but not calls to a Lichtenstein or Estonian premium number.
It makes a huge difference.
In Europe calling a Base number is covered by new EU regulation.
Which is not the case for non European premium numbers.
Those premium numbers offer revenue sharing on their termination. EU 'legit' mobile carriers DON'T.
Base does not share any revenue on its termination. ZERO.
Concept of FREE is a concept. Subscriber always pay somehow.