Here's the link for the script mentioned. I tried and it worked like a champ. (You can also search for "Protected AAC" in the 'kind' field, but of course it'll also find songs you yourself purchased.)
Be sure to throw this guy a couple of bucks for his time. This is a much-needed option for the Mac user community (no, I don't know him, I'm just a user like you who's P-O'd at having to spend a bunch of time deleting tracks so my IPod will sync without me having to click through a thousand nag screens bugging me about DRM. Not that I needed much more encouragement to flee ITunes Store for Amazon's DRM-free downloads.)
http://dougscripts.com/itunes/scripts/ss.php?sp=trackdownpurchases