[razfaz]

>Are Simyo and Blau more widely available than the Aldi Talk SIMs?
They'll be sent to your residential address via mail, so no they aren't.
>Any chance of finding Simyo or Blau at TXL?
Unfortunately chances are close to zero percent. If you have any chance, look for a Aldi store. The SIM can be activated later on over the internet. If you don't need GPRS or UMTS, there are many other choices. You can't be sure to get an UMTS capable SIM at Simyo or Blau, while Aldi only sells UMTS SIMs.
>So if I buy a SIM in that country, I should just be able to dial the number after the country and area or city code right?
Correct.
>Is there a harm in dialing the full number with the +49 (which is for Germany)? Will it make the prepaid SIM think it's doing international roaming?
No, you can either dial +4930xxxxxx or 030xxxxxxx all the same at the same costs with a german SIM
>Also, when can you omit the leading zero in the country or city code and when must you include it? Or is it better to include it all the time?
You always skip the leading zero in the (german) city code or it won't work. Don't skip anything in the country code. you either dial +4930xxxxxx or 004930xxxx

Anyway, have fun

Just checked on the blau.de homepage. They'll only send UMTS USIMs to all customers that order on that page.