Um my phone is defo a computer. Look up nokia N900
There may be some cross over between the two, but I can't see a phone as a substitute for a computer.
Checking email? Seems valid enough. At least if you can avoid having to access a web page.
Isn't text messaging supposed to be the way to go with phones? I'll leave that to people that want to and can master those little keys (I'd rather talk directly or leave a voice message that will probably be ignored).
Using the internet? Maybe you want to do this, but with their tiny screens and limited resolution I'd rather avoid the eye strain.
Apps? Lots of them, plenty of poor ones (along with more than a few games I've got no interest in), but I'm not doing my word processing on a phone (not if I can avoid it).
Is everything with a microprocessor a computer? If not, is everything with a microprocessor that can do some of the things a computer can do a computer? What the hell is the iPad? And finally, do I want all these devices (cause the phone isn't going to make a adequate replacement for all them yet)?
Far too many question, You keep thinking of your phone as a computer. I'll keep wondering when somebody's going to tell me their wristwatch is a computer.