How about this:
Create your own plugin that configures WordPress and installs other plugins programmatically. Now the question is: how does your plugin get activated automatically?
How about this: in functions.php of TwentyTen hook into "init", and activate your plugin there (if it's not already active). This should only be a few lines of code in functions.php. Your plugin takes care of the rest of the configs and installs.
The MultiSite solution sounds good, but in this case you'll be the real owner of the sites -- not your clients. You'll be responsible for them even after project delivery. This could be a business opportunity, though. Charge clients a subscription fee in exchange for keeping their websites always up to date, optimized, and secure. But, in this case, you'll have to look into how to handle requests of withdrawal from your network -- you may find yourself again compelled to create an automation script for that :P