Currently Being ModeratedMar 26, 2012 10:16 PM (in response to brevemike)
Yes, installing the OS X Lion Update 10.7.3 (Client Combo) will reinstall the App Store for you.
Restart your Mac after the combo is installed.
If that didn't help, some third party apps like Little Snitch can prevent the Sign In window from appearing > Mac App Store: Sign in sheet does not appear, or does not accept typed text
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