Is it possible to have different pricing structures for different albums, or to turn off Google Checkout completely for some albums?

Here's why:
I have 3 albums:
One of them is clothing design, where I'd sell t-shirts.
Another is fine art prints, which I'd sell by size.
The last is a design portfolio, where I wouldn't want to sell anything.

Is that possible? Or, even better, to only let certain pricing structures show up for certain tags? (so, the tag "t-shirt" would pull up the pricing structure for it, and "print" would pull up the structure for that)

You can pass price lists directly to the checkout function and of course can tie that together with certain tags. More info, you guessed it, on the functions guide: http://www.zenphoto.org/documentation/plugins/_plugins---GoogleCheckout.php.html

That link is no longer working, does it live somewhere else now?

That plugin is not an official one anymore. We still host it for download but it has not been updated for over a year:
http://www.zenphoto.org/2008/06/zenphoto-plugins/#GoogleCheckout

For the info you have to look into the file comments itself.