SafariTabs and Safari 5
In short, I'm working on it.
For anyone interested in a little bit of history, when Snow Leopard (OS X 10.6) was released, which included further restrictions on InputManagers resulting in SIMBL having to be rewritten I was all set on discontinuing the development of SafariTabs. The reason being because its feature set was reducing with every release of Safari and to be honest I didn't like the new approach SIMBL was taking to 'injecting' plugins into target applications.
As well as the sheer number of requests I got about SafariTabs support on Snow Leopard, I quickly realised that I couldn't live without being able to undo the most recently closed tab, which is why I continued its development. I've always maintained that SafariTabs was created to add features and functionality to Safari that I personally found lacking and is the primary reason why I've resisted in adding every feature request that I've received. Much like the UNIX philosophy, it is intended to do one (or a few) things and do them well.
With the release of Safari 5 Apple has finally added an extension API, tab undo support and the ability to control how links that open windows are treated (for example, automatically or always open them in new tabs). The last 2 features being those that SafariTabs provided and seeing that I rarely use the tab restore function I was again debating whether or not I should update it. Similar to when Snow Leopard was released I've since received a load of requests for Safari 5 compatibility, either because users genuinely use the tab restore or they've yet to realise that Safari now provides half of the features that SafariTabs once did. Regardless, there seems to be demand for it and so I appreciate your patience, while I work on updating it. I'll be releasing a compatibility (and reduced feature set) update within the next couple of days.