Q: Screensaver currupts login screen?

Qustions:

Which iMac do you have? > How to identify iMac models

How much RAM does it have? > Does my mac need more memory?

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Thoughts:

Complex Screensavers use lots of RAM and can cause problems for systems that have a minimal amount of RAM to run the OS X. 

Try disabling the Screensaver for a time by setting it to Never and use Display Sleep instead to see if that corrects the problem.

If your iMac only has 2GB of RAM or even 4GB for some, then you might want to upgrade your RAM. Because to me, it sounds like Lion is struggling to recover RAM from the Screensaver at login.

I believe this describes the problem.  The solution is at the bottom and relates to automatic logoff.

http://support.apple.com/kb/TS4135

Good one captfred, that's the first time I've seen that article.

I'm a little surprised, it reminds me of the old Built-in display dims before sleep problem way back in Tiger that didn't get fixed until some time late in Leopard or early Snow Leopard.

Only worse.

Thanks, I found it last month in the new article listings..  good place to peruse now and then. 

http://support.apple.com/kb/index?page=articles

Thanks