As a Canadian, I can't use Google Voice in order to send my SMS. However, there are numerous apps that act very similarly, including the new DeskSMS from the developer who brought you ClockworkMod.
DeskSMS transfers SMS to both your e-mail and other IM applications through the use of your Google Account. This allows you to answer SMS on-the-go and with the use of a full keyboard. As someone with big fingers, I can't tell you how annoying it is to try to do a large amount of typing with an on-screen keyboard.