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I did this on my Mac with excellent results, although I found the Screen Capture option on the Device menu, rather than the Actions menu as Chris suggested.
After you download the SDK, turn on USB debugging on your phone by going to Settings -> Applications -> Development then turn on USB debugging as shown in the screenshot below (you can see I got it working :-)
Hook up your phone to your computer via the USB cable.
ddms (the Dalvik Debug Monitor) is started by a shell script found in the SDK's tools folder. Even if you're not a developer, it's not a problem to use it. Just double-click to start the script, which starts the program. Ignore the various messages in the textedit window - "these are not the droids you are seeking" Your device should show up in the ddms window, as shown in the screenshot below. Ignore the various debugging messages in ddms - these are not the droids you are seeking either.
Go to the ddms Device menu, choose Capture Screen (or CTRL-S) and you'll have your screenshot. OK. It's not user-friendly, but it works.
The other option that I found suggested on the net (but did not try) was to "root" your Android phone (i.e. make it possible to give applications root access) as described here: http://smarterware.o...r-android-phone and then download an app like Android Screenshot from the Android Market. There may be some security risks to this approach, since you are giving total phone access to the screenshot app, and it may well be that one or more screenshot apps is doing something scurrilous without telling you. I haven't researched any of them enough to vouch for them.
