This ability comes in handy occasionally (rarely) for me, but I just use Verizon's email-to-SMS service and procmail. What would get me to switch to this, though, would be if you offered a command-line of sorts that lets me control it by sending it SMS messages. For example, let me tell it to start forwarding or ignoring emails that match certain rules.
i don't know what teleflip does, but this app has started out of a basic need: when i'm away from my computer, i would like to know if i have received any (important) emails. right now it truncates an email to 140 characters, so you can't read the entire mail (most of the time) -- just know the sender and subject and the first line...
btw it only works for cell phones in the US.
and on popular carriers...
(using the email-to-sms interface the carriers provide -- for example if you send an email to PHONENUMBER@tmomail.net, then a sms message will be delivered to PHONENUMBER) see the lifehacker link that sam odio provides above.
if someone wants to work with me on this, email me (sri at defcraft dot org). work to take this public (right now it won't handle a massive load) -- it is build with ruby on rails and python, so if you have productions experience with rails -- that'll be helpful.