I think I figured out what's causing Google Desktop to go nuts and consume 400 megabytes of memory on my system. It's the download manager Reget Pro. While it's downloading a new zip file at 120kbytes/sec Google Desktop consumes 30kbytes/sec of memory. When I quit Reget Pro Google Desktop stops consuming memory entirely. Memory consumption seems to be proportional to download speed, too. There's some garbage collection that happens afterwards so I'm not sure if it's a persistent leak, but it doesn't look right to me.
User support forums have some information about Google Desktop having trouble with corrupted zip files. Reget is downloading multiple sections of the file at once, making for a weird looking zip during the download. So maybe that's the problem.
Google is so big now I don't even know how to file an effective bug report for this. Kind of hoping someone who works there reads this blog post. I also tried the official support forum.