Had another go at the tab thing tonight, trying to get it a little more "aqua"... The result is attached.
Both Allan and myself have pretty much turned down the idea. Whilst I think they look great, they're a little too small, and the X and dot don't stand out enough... I'd probably have to start again with a new (slightly larger) "red ball" (this one was borrowed from OmniWeb).
But these flat crosses are becoming a little bit of a de-facto OSX standard in one form or another -- most tabbed apps are using them, so I figure there's no need to rock the boat.
We're also ummming and ahhhhing about coloring the selected tab from white to something else (like an Aqua blue), but my preference is definitely white.
For what it's worth (Drew, et al), yes, I'd love to have the selected tab bleed into the document, but Allan doesn't dig it, and it does have some issues in regards to the colour of the document (eg Ruby's black) which are a not exactly simple to work around. One day maybe :)
Justin