Well, if last year is anything to go by, then there'll be a 2009 and 2010 montage. Now, considering both this year and last year's E3 conferences were considered disappointments by Iwata, I'd expect a good October conference. Last years conference had some very pleasant surprises, and most of the announcements came out of nowhere. Sin and Punishment 2 and Punch Out were big surprises.
I expect the 2009 montage will contain no new titles.
As for 2010, I think we'll see footage of things like Monado, Galaxy 2, Cosmic Walker and other announced titles. Now, there very well could be a Zelda Wii trailer. But I'd expect it to be nothing but a 1 minute teaser, and I'd expect it to be shown if the game is still on course for a 2010 release. Now, I think it's likely we'll see a couple of new titles from existing IPs. Star Fox, Donkey Kong, Kirby and F Zero are likely suspects from franchises that have remained fairly active. I'd expect one or two from that list to be announced.
I don't think Kid Icarus is going to turn up, but if there is a retro revival, I'd guess at Pilot Wings. It is the perfect fit for Wii and could be a big success with the new audience Nintendo is tapping into. I think another possible reveal could be a new Wave Race-another game that could satisfy Nintendo's old and new audiences.
(I'm clueless when it comes to what will be announced for DS)
Guessing what Nintendo will do next is extremely difficult. I think any of the predictions in this thread are plausible scenarios for the conference. Nintendo have plenty of products in the pipeline and have plenty of IPs to draw on, but it's very difficult to predict accurately what they'll decide to show.