While I was within Safari, I was showing someone what the expand gesture was like for iPhoto, and discovered that the new trackpads resize the page font using that same gesture. Further investigation found that Mail does the same thing, and Preview resizes the whole page/photo.

[robg adds: According to a friend with a new laptop, the Preview zoom feature is shown in one of the how-to-use videos for gestures, but the font resizing functions are not. If this turns out to be clearly documented somewhere, please let me know and I'll remove the hint...]