I know you say you don't want another line.... but... this is what I have:
I have one line (not my main # ) on the Iphone 3g with the appropriate iphone plan.
I have another line (my main#) on my Bold (formerly was my curve) with a BB plan and a sms plan.
When I want to use the iphone for an extensive period - I swap the sim. My main line is now connected to my iphone 3g. The only sacrafice is visual voice mail, everything else works!
But....since I got my Bold...not sure I'll want to use my iphone very much