A few more notes on this:
1. You can work around it by going through a reformatting proxy such as Skweezer or Google Mobilizer. It's a pain in the butt to have to do so though.
2. This is affecting me on a great number of URLs, not just the two I posted.
3. I don't see any correlation to CSS @import specifically.
4. It seems like all WM5 Smartphone devices, not just the Dash, should be affected. Is this true? Any non-Dash Smartphone users willing to try one of these URLs?
5. I really wish I could report this bug. But who do I call? T-Mobile? I don't really want to spend an hour with someone telling me to reset my phone before I can convince them that it's an IE problem. And then what? They report it to HTC who reports it to MS? I'm sure that's going to work out. :?
6. I tried Opera Mini, but it only worked the first time I launched it after installing it. Subsequent attempts to launch Opera Mini fail with some sort of weird midlet error.