The ability to include (and be able to switch to) currently open tabs among the suggestions in the awesome bar is a long time idea that now looks closer to becoming a feature thanks to an experimental build by Blair MC Bride that adds this functionality.
In brief, you enter a few keywords as you usually do to get a suggested site, but currently open tabs that match those terms are listed as well and marked with a tab icon. You can then select either the tab to switch to it, or the URL if you want to load the page in another tab.
It works pretty well for me but as Blairs tells in his post, there are a few issues to resolve before going for code review and getting this checked in to the trunk (Minefield). There is no official say, but I’m betting this feature will be available in Firefox 3.7, hopefully with an operator to limit searches to tabs for quicker finds.
If you know how to handle the risk of messing really bad with your Firefox profile or won’t miss your computer that much, you may want to try the experimental build.