Thanks again for the comments.
Derek, the tracking with the 12" Dobsonian is OK for planets. Here's a video clip used to create one of those images in the Saturn pic (14:04UT):
http://www.youtube.com/watch?v=T_KHeyO_P24
The movement you see is typical of the tracking. About 60% of the time, at this magnification, I have to correct manually during the exposure (using "rate 2x"), and if I do that I have to anticipate for a very short moment of non-tracking before the tracking kicks back in (like a backlash of sorts).
Also, I have found that you need to wait a minute or so after slewing to an object before the tracking movements calm down into somethin more regular and accurate.
It's definitely not something you can set and forget, you need to keep an eye on it. The Jupiter image above was shot with no auto tracking but using manual alt/az movements of rate 1x.
Hope this helps.
Cheers,
Tom