You'll need to distinguish between new tab and new window won't you? Certainly for today's apps; I do the two things at very different times.

I like the first one for new window, but with just window in the icon not too. Then, why not make new tab the same but with just the upper half of the window, kind of like the one just beneath it (but with the rounded corners at the top)?

Man it's hard to use words to describe spatial things (and I just recently found out why, thanks to Steven Pinker!).

Thanks for the feedback
I guess you're after something more like this?
newtab_cursor.ai @ 200% (RGB/Preview) on Skitch.com by @keith

I like the top-left one the most as a "new window" icon, because it has a + in it (pretty standard on iPhone/Mac as "add") and it looks like 2 windows. I'm not sure how I would distinguish between "new tab" and "new window" in a cursor.

Thinking out loud:

In my regular browsing, the only time I know I want a new window to come up is for the "bookmark on delicious" bookmarklett I use. It pops a new window where I can type in info about the page I'm looking at, then it closes when I hit save. It's better as a window than a tab, because I can read & copy the page under it.

Extrapolating from this, when clicking on a link:
* replacing the current page = following a thread
* new tab = forking a thread
* new window = comparing something to the thread.

But that's just based on casual observation of myself, and I've been known to miss the obvious there .

Bit slow to the party, but I think you need the multiple windows to make it clear. The + by itself says zoom to me. Another option might be something like an external link icon: http://bit.ly/CFQJT