When that happens to me I think its because you may already have it..or if you have it in titanium the market updater may be affecting it? Just something I rebooted and played with the tb app a bit and fixed it..been a while though..good luck!
I think it's a bug with the Market. I have seen a lot of weird things with the Market on builds that worked 100% for weeks. For instance, I have 2 apps right now that are in my "updates" section, but when I click on the app it gives me "open" or "uninstall", no "update" option.
I think they've been doing some work trying to improve the market, and are temporarily breaking certain things in the process.