iTunes "Check for Update" button puts up a dialog box asking for a file This is on Snow Leopard.

When I plug in my iPhone or iPad into iTunes and click the "Check for update" button, it puts up a dialog box asking me to choose a file. This is supposed to happen if you press option while clicking Check for Update [which I'd done once], but now it happens every time. Restarting iTunes makes no difference, and it happens both on iTunes 10.4 and 10.5.

How can I get iTunes to check with Apple for an update? Thanks.
I am on Lion but had a similar problem. Based on info here: http://developer.apple.com/library/ios/#featuredarticles/FA_Deploying_iTunes/Introduction/Introduction.html , which says that 'the file selector dialog only appears only if automatic detection of iOS updates is turned off', I tried removing ~/Library/Preferences/com.apple.iTunes.plist.  

This fixed things for me.  Quit iTunes before you do this, and move the plist file somewhere so that you can restore it if necessary. (It may be possible to change this preference setting without messing with the plist; however, I couldn't immediately spot where to do it in the UI).