I recently acquired an 3G iPhone just to check it out and write some software. I must say I'm already totally addicted to it. I'm sorry to say that but the N810 is no real competition. Stuff that really get me excited: really mobile email, very fast GPS (and maps), excellent connectivity (WLAN + 3G). The N810 has all these features too but just not in the same quality. Maybe I would be less impressed if the N810 had GSM/UMTS connectivity. The I only have to carry one device and have connectivity in most places factor just kicks ass.
Some points where the N810 still is slightly ahead: web browsing (big screen, flash), the hardware keyboard (nothing beats this on a shell), and of course the open source platform.
I'm sure that I will continue using my N810 for specific tasks once in awhile.