As you probably know, there are somewhere around 100 000 apps currently available in the Android Market, growing faster and coming forcefully behind Apple's App Store (which has around 200 000 apps). Is isn't currently any problem for Apple, but there's a matter of time until there will be a real challenge for Apple to keep up with the incredible grown of the Android Market.
However, the Market (the place from where apps for Android are officially installed) isn't a perfect app. Actually, it's quite buggy, maybe too buggy in some situations. Nexus One, a device on which the Market worked perfectly and which is considered "the Google Phone", got some problems since it was updated to 2.2. There weren't big problems until about a month ago, and, from then, my phone fails most of the times to install some updates (when more updates are available in the same time).
This is a common problem on many Android devices, and here is what you can try in order to get Android Market working properly. First, disconnect your device from wifi. Then go into Settings | Applications | Manage Applications | All (if these steps doesn't correspond to your device, just find in the settings the list with all apps).
Now, just don't force it to update everything. Update every app from the update button (inside the app page), and make sure you don't update two or more apps in the same time. Otherwise, it will continue failing to update some of the apps.
Keep in mind that this is just a temporary solution, and doesn't work at all times. Additionally, make sure you have enough free internal storage memory on your device, as this could be the problem in some situations.