Last time I checked, my iPhone can't play DVDs, doesn't have flash, can't hold 300 GB of music, can't edit my photos, can't play the full version of Spore, can't create a MS Office file (word, excel, database) and can't burn CDs.
So no, it's not replacing my MBP anytime soon.
However, when I had to get my computer repaired, my iPhone kept me from going into internet withdrawal. It also kept me from getting lost when I was in Houston for the Green Party convention.