Yes, most of the stuff is just upgrades. There's a little bit of new stuff for IPv6 and WiMAX, but it doesn't add much weight (maybe a couple of megabytes on the high end).

Nope, sorry.

OS2007.x/maemo 3.x = Bora
OS2008.0/maemo 4.0 = Chinook
OS2008.1/maemo 4.1 = Diablo

We know at least one more codename for sure, Elephanta, and the one after that is maybe, Fremantle. Presumably Elephanta will correspond to OS2008.2/maemo 4.2 and Fremantle OS2008.3/maemo 4.3 (Edit: Jaffa may be right about there being no 4.2 or 4.3 and Elephanta being OS2009/maemo 5.0, but I'm going to play the pessimist here and he can play the optimist—or he's the optimist and I'm the pessimist . . . depending on how you look at it ), or maybe Fremantle will be OS2009.0/maemo 5.0, no telling yet.

Fun Facts for Additional Obfuscation: OS2005 didn't have any (known) codenames, OS2006 had three (one for each point release), Mistral, Scirocco, Gregale, OS2007 had only one (for all point releases), Bora, and now Nokia's going back to one for each point release with OS2008. Who wants to take bets on which way they'll go for OS2009? I'm in favor of big cats for everything second point release.

Edit: Others were faster on the trigger, but I win for completeness.