Middle clicking a hyperlink does not open a new tab for pages with frames, e.g. hyperlinks in a Gmail message. A reproducible example is to go to http://www.reddit.com and click "comments" beneath a link to go to the comments page. In the side bar on the right, click "toolbar link," which will display the page with the Reddit toolbar at the top. Middle clicking any hyperlink on the page won't open a new tab. Example page with Reddit toolbar: http://www.reddit.com/tb/991ft Clicking the X to close the toolbar and reload the page normally will allow hyperlinks to open new tabs on middle click. Example page without Reddit toolbar: http://www.mongodb.org/display/DOCS/Home Tested in Safari 4.0.2 and WebKit nightly r46919 downloaded on August 9th.
Couldn't reproduce in Safari 5.1 (webkit 534.51.22) or Chrome 16.0.904.0 dev-m (webkit 535.7).
I do not experience this in Safari 5.1.1 either. It is now unreproducible and has apparently been fixed.