I currently have a discontinued Nokia 8110 GSM Phone that operates
solely on the 900 mhz radio frequency.  It was made for usage in the
UK under O2/Vodafone and it is curerntly unlocked. (However, my AT&T
Wireless SIM card does not work, it says "SIM Card Rejected", but my
Cingular SIM card registers.  These two sim cards are
pre-Cingluar-AT&T Wireless Merger SIM cards, but this issue is really
a secondary issue).

I currently live in the USA and would like to know if there is some
way to hack, mod, tweak, and/or alter the phone's capability (probably
the transceiver) to function on the 850mhz/800mhz frequency so it will
be compatible with the domestic GSM networks?
I'm not the best with electronics, but I'm willing to attempt the job
myself.  However, if there is an outfit that performs this kind of
service, that would definitely suffice.

Hi,

Take my advice:
IT WILL COST YOU ***A LOT*** MORE TO CONVERT YOUR PHONE THAN TO BUY A
NEW DUAL BAND PHONE. IT IS POSSIBLE, BUT VERY EXPENSIVE. BASICALLY, NO
USE.

KIND REGARDS, 
COMUGENIUS

OK, since you didn't answer the question, I won't be charged (except
listing fee), right?  That's fine.  I appreciate the advice. 
Basically, it wasn't necessarily about getting my phone to work just
because I didn't want to buy a new one.  It was more about having a
novelty phone that is a classic and was featured in the original
matrix movie and being able to have a working "matrix" phone in the
USA.
Sounds silly, but definately not worth hundreds of dollars.