Tomorrow is the big day: the invite-only Google Android event. While we’ve all pretty much assumed they’re going to tout Android and reveal the Google-sold Nexus One phone, there’s always the possibility that Google will surprise us all.

Google’s Nexus phone was first “leaked” in December after they distributed them at a Google party (no gag order in effect). Naturally, the phone created a free social- and mainstream media frenzy in pretty much no time flat.

But employees were the only ones with sneak previews—until recently. Saturday, Engadget posted a video tour of the Nexus UI and a preliminary review—and the Nexus is not the iPhone killer:

[I]s this the be-all-end-all Android phone / iPhone eviscerator? In two words: not really. The thing that’s struck us most (so far) about the Nexus One thus far is the fact that it’s really not very different than the Droid in any substantial way. Yes, we’d say the design and feel of the phone is better (much better, in fact), and it’s definitely noticeably faster than Motorola’s offering, but it’s not so much faster that we felt like the doors were being blown off. It is very smooth, though we still noticed a little stuttery behavior (very slight, mind you) when moving between home pages. Still, opening applications and moving between them was super speedy, as was Google maps, and any area of the phone where you’ve got to get through long lists. Don’t get us wrong, the phone cooks — but it’s not some paradigmatic shift for Android.

So while it’s slick and cool, the Nexus isn’t a huge technological change—and it’s not changing the game the way the iPhone did. It’s a viable smartphone option, and probably typical of the second generation of Android devices.

And then there’s the possibility Google will surprise us all. At Search Engine Land, where Danny Sullivan will be liveblogging the event tomorrow, Greg Sterling points out that if Google really wants to surprise us, they could introduce an Android tablet computer—effectively upstaging Apple’s event scheduled for January 26, where they’re anticipated to unveil their own tablet.

Boringly, Google will probably just formally premiere the Nexus. Maybe they’ll be ready to sell it immediately, even.

What do you think? Will Google go for the obvious or the surprise factor?