This is an incomplete but interesting starting place:
[link|http://sms.bigfoot.com/index.php?pages=networks|Bigfoot SMS partners]
Bigfoot.com is offering SMS text messaging from a PC to most phones. I might be interested in using it if I knew more than one person who uses SMS. What makes it interesting is that they list the technology and frequency (in hundreds of MHz), although it's not complete (e.g. they miss the biggest US carrier, Verizon).
Also, I should note I'm really not a cellular geek; OTOH, I do have an uncle who's spent over thirty years in radio and cell phones.
Tony