AT&T over in the States are reportedly talking about bringing out the iPhone for enterprise users. The analysts quoted in this piece certainly don't seem to think it's the brightest idea, because:
a) It's touchscreen so you can't use it while driving (er, so's a Windows PPC, and you shouldn't be using it while driving anyway).
b) It doesn't have a removable battery (not an issue for everyone, but fair point).
c) Apple hasn't built a phone before (true-ish - but why do you think it briefly teamed up with Motorola for the ill-fated, storage-challenged, iTunes-centric ROKR phone? Ahhh....).
d) Enterprises won't be able to write so many apps for the OS X-based operating system (a valid point).