I use an old PowerBook G4, and though I love a mouse sometimes I'm at a place/position that doesn't easily allow me to whip out my Wireless Mighty Mouse and get to work. I've gotten used to the trackpad and though it's not as nice (to me) as a mouse, I can make do. But there was at least one shortcoming that always bugged me until recently.
Specifically, with a mouse you can double-click a word to highlight the whole word, then while continuing to hold the button down, you can drag the mouse left - right - up - down to select blocks of words/sentences in entire groups, selecting whole words at a time. Even with all the settings checked on my PowerBook's double-tap-enabled trackpad, I couldn't get it to actually perform this maneuver.
Recently I discovered that you can indeed get the same behaviour out of the trackpad, though it's not documented that I could find, and unfortunately it's a two-step process. First, you have to double-tap the word, which will highlight the selected word. Then pause very briefly and double-tap the same word again and immediately start dragging. Now as you're dragging, your selection will automatically select entire words instead of the usual behaviour of just up to the pointer. Be careful though -- it's a two-double-tap process, not one-four-tap process (a four-tap selects the entire paragraph). So make sure you put enough time between your double-taps so that the system knows the difference (.05 of a second or something).
I also discovered that if you have "dragging" enabled but not Drag-Lock, and you find that you don't want to wait the half-second it takes for the system to realize you want it to release things you're dragging after you take your finger off the trackpad, simply give it a quick tap once you have dragged the item to where you want to drop it. Anyone that's tried the Drag-Lock option knows that that's how the system works with that selected: the OS waits for you to tap the trackpad to signal "release" of the drag. This hint is basically pointing out that the behaviour extends to systems with only "Dragging" enabled. This only saves a quarter-second at most, but sometimes it's more than anything an irritation-saver.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070925152545926