I found this issue, but have been having trouble navigating Microsoft's site to report it. Figured I would at least post it here so others would be aware if they are experiencing the same issue.
My Setup:
OS - Windows 7 Enterprise x86 SP1
Applicable Software - Microsoft Office 2007 Pro/Plus SP3
The issue is as follows:
You have more than one Excel file open at the same time and only the most recent file can be the top most window. You can de-maximize the window to see the other documents are open, but you cannot make those documents the top most document within the Excel window even when they have focus for data input.
The issue does not appear to happen to all systems in my environment that have 2775511 installed; however on systems that are exhibiting the issue, removing 2775511 resolves it.