From what I can see from an article on GSM (Wikipedia) GSM coverage is available throughout most of the United States. http://en.wikipedia.org/wiki/GSM
Interestingly there's a map of GSM coverage and the US is well represented http://en.wikipedia.org/wiki/File:GSM_World_Coverage_2009.png
I now have a replacement GSM UMTS W-CDMA phone ordered from Queensland but delivered in one day from Hong Kong. It's not locked to any particular carrier and entered the country legally.
Although it has UMTS features (3G) I've set the phone to GSM only as 3G seems to drain the battery given all the things UTMS support. It's only a flick of a switch to turn 3G back on if I need it - so nothing is being lost.
Wikipedia also have a page illustrating the advantages/disadvantes of the various mobile/cellular phone standards http://en.wikipedia.org/wiki/Comparison_of_mobile_phone_standards
The major mobile standards listed are GSM UMTS(3GSM) and CDMA (used in the United States ). CDMA doesn't use a SIM card.
I believe the system whereby you enter a phone command using *# (there are many of them - most undocumented ) such as *#06
# mentioned earlier is unique to GSM (Global System for Mobile Communications).
I guess if you actually wished to purchase a GSM phone there would be someone somewhere ready and willing to export one to the United States. Not living in the US I can only speculate where you'd be able to purchase a suitable SIM card.
As for the IMEI number I find this is a number printed on the box the phone comes in. If your phone came in a box perhaps you could find the equivilent printed on the box as well :-) Sure beats poking around inside and looking for obscure stickers underneath the battery :-)
BTW: Someone (anonymous) has added a comment to my July entry regarding Theo Bee's (LTUAE) passing.
I tweeted/twittered reference to it yesterday http://twitter.com/bob_bain/status/5956739655
Comment to LJ entry (Jul 29 2009) "Science is nothing but perception" http://tinyurl.com/y8fysg7
added to http://tinyurl.com/yh23bjn