Actually Sega and Nintendo have been friends for almost 10 years now, i mean Sonic Advance came out in 2001.
Personally the new console wars aren't that different, although it seems that to many fans Nintendo are the new Sega, in the sense that they can't seem to put a foot right. Although i hate the idea of console wars and just wish everyone would just get along and enjoy what they have instead of having a go at what other people have chosen to get.
As for sega re-entering the market...well it would be nice i admit, but it would be a bad move on their part, they get far more money now then they did before, going multi-format have allowed them to get to more audiences, get back their former reputation (unless you mention Sonic, it seems they have a lot to make up with him, Sonic 4 is a great step though) and make up for their many mistakes.