Russel's concerns are what I was trying to get at. Being able to exchange SMS with cellphones is great, but there are many services that send text messages that just don' work with non-cellphone SMS (and apparently some cellular carriers.) You don't get any errors when you enter a number for these services, you just don't get the message. If all you use SMS for is texting friends, then great. But for most purposes, I find I need to use my "real" cellphone number or I won't get the message.