I used to think so when I had my Nokia, since text input was a pain, even with T9 active, but with the iPhone, I get along fine without it. As it is, I already carry around my headphones, and my BT headset, and a charging cable (need another AC adapter for it).
Considering I've used a BT keyboard on other devices, I thought I would need to have one for the iPhone. Fact is, it doesn't bother me not to have one. I find that I like the convience of being able to type out something quick without the need to carry one more item with me.
It's not something I'm really interested in. One of my main motivations for buying the iPhone was to DECREASE the amount of crap I carried around in a day (missioned accomplished, btw, and not in the GWB sense). A BT keyboard would just be something else to manage. YMMV.