This has been around for a few months now, but worth highlighting if this sounds like a scenario you are facing.  In the Summer ’12 release, one new feature was the ability to ‘log in as any User without first having access granted’. This was in response to a previous change, which limited the amount of time you could grant access to 1 year.

If this feature is something you would like, just drop Salesforce a line and they can enable it for you!

See a Salesforce blog listing of this feature

http://blogs.salesforce.com/product/2012/06/log-in-as-any-user-without-first-having-access-granted-new-feature-in-summer-12.html

Grant access to all users