Actually, instead of using the Sense as default, set "Home" as the default. The Sense is actually a bloatware rather than a launcher itself. Usually, this only happens when there are two launchers available. Do be careful with launchers. Uninstalling them is not impossible, but your phone may terribly crash with improper uninstallation (and if there is no valid launcher installed thereafter).

Do you have root on your phone? Are you running a custom ROM? If your answer is yes to these two, that will make your life a lot easier actually.

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Added on September 6, 2010, 10:34 pmI'm subscribed to this thread and will see how it goes Usually there is a way out of Android when something like this happens.