Personally, I bought a 3GS in the middle of December. Iirc, I kenw abotu the January event already, but I certainly new a new iPhone would come out in the summer.
However, I didn't upgrade from an earlier incarnation of the iPhone - I had an LG Shine. If I had already had an iPhone, I imagine I probably would have waited for the summer, but since I didn't have an iPhone, and I really wanted one (never even owned a smartphone before) I think I made an amazing choice.
I must say though, considering how much I'm enjoying this phone, depending on the upgrades in June/July, I may even upgrade then after selling my 3GS.