Yep happens to me most of the time
From Google:
Some users have reported suddenly being asked to sign in to an @youtube account when trying to access Android Market. We're looking into the issue, but in the meantime, you can take the following steps as a temporary solution:
From your home screen, visit Menu > Settings > Accounts & sync, and you may see a @youtube account listed. Tap the account, then tap "Remove account" at the bottom of the screen. If you don't see an @youtube account listed, not a problem, you can still proceed with the other steps outlined below to resolve the issue.
Clear the cache and data of the Market app and the Download manager. Here's how:
Visit Menu > Settings > Applications > Manage applications
Tab over to "All"
Select the Market app, and then tap "Clear data" and "Clear cache"
Select the Download manager, and then tap "Clear data" and "Clear cache"
Temporarily uninstall updates to Android Market by following these steps:
From the home screen, go to Menu > Settings > Applications > Manage applications
Tab over to "All" (or, if your device is running Android 2.1 or below, press Menu > Filter > All)
Scroll down and select the Market app
Touch "Uninstall updates"