True, but as the SIM-plus-phone packs can cost as little as $15 in retail stores, it might be a quicker and easier option to do this.

There is a 100 or 200 MB option for about $25. I used it in Boca Raton, Florida last May.

One other option for you is T-Mobile USA, although its 3G frequency is incompatible with most 3G phones. They have a day pass for Internet access that is quite attractive.

What we were saying was:: "Good luck finding one!"

They have a lot of better things to do with their time than finding a 'SIM only pack'. Stores may or may not have them. In my experience they do not--almost never.

You might get $30 .... 4g t-mobile plan and get 30MB web. Speed may slow after 30MB rather than unavailable. I would check that out. If just email this may do it.

So my wife went into a AT&T store and they told her it won't work - apprently they even tested it and it didn't work.

Not sure why - HTC Desire is:

HSPA/WCDMA: 850/2100 MHz
GSM: 850/900/1800/1900 MHz

Isn't the above compatible with AT&T GSM?

@alect
AT&T runs GSM and UMTS at 850 and 1900 MHz, so your HTC Desire is definitely AT&T compatible. Since it supports GSM at 850 and 1900 MHz it will work on their 2G network and as you obviously own the version sold by Telstra (Model A8183), which also supports UMTS at 850 MHz it will even work on most of AT&T's 3G network. Only those 3G cells running at 1900 MHz will be inaccessible to your phone, but these places will surely be covered by 2G. So the only effect of the lacking UMTS1900-support is that at some places you won't get 3G and thus no broadband speed. But that's not an issue for voice calls at all.

So what is their problem? Is there a data issue? HSDPA?

They are just incompetent I guess. AT&T use exactly the same network standards as the rest of the GSM/UMTS-world including HSPA. Even if your phone wasn't compatible with their 3G network (e.g. if you had a European HTC Desire which supports UMTS only at 900 and 2100 MHz), it would still work on AT&T's 2G network.
There shouldn't be any issue except for a slightly inferior 3G coverage due to your phone not supporting UMTS at 1900 MHz.

That's what I said to my wife. Apparently the guy in the store put in a demo prepaid SIM into the phone and it didn't work so they told her not to buy since there is no refund and it doesn't seem to work.

Is it just a matter of manually rescanning networks to pickup AT&T?

It should find the network even automatically, which may take a bit longer upon the first use. I recently read of AT&T SIMs being tied to the phone, in which they are used for the first time, for six months - if that is true that perhaps is the reason your phone couldn't register with that certain SIM.
Lastly you should rule out that you phone has a SIM lock. But the guy should have seen and told that if that was the case.