You can try the manually downloaded version of the update. While I don't necessarily buy the superstitious reasoning some people have that claim them to be more reliable, since AFAIK, no one has actually done anything even remotely resembling scientific testing to try and show that it couldn't have been any of the other possibilities... It seems to work for some people. I may not buy the reasoning people use, but it is hard to argue with results.
Just go to Apple's support page, and from there you should be able to find links to get to a downloads page, and you'll find the 10.6.8 updates in there.
Updated my MacPro to 10.6.8. After it almost completes the boot process, it just shuts down. In safe mode, it completes and I can login in, but within minutes it just shuts down. System log suggests something to do with the default vpn client, but I tried removing it from start up and it still happens. Used Time Machine and install DVD to revert back to 10.6.7. Would like to get the issue resolved so I can upgrade to Lion.

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