This blog post contains a bookmarklet to force links in Safari on the iPhone to open on new pages. As noted there:
After experiencing and thoroughly enjoying the iPhone for the weekend, I've found that one of my biggest gripes is that there's no "Open in New Tab" command, which makes all the sense in the world since there is no right click anywhere in the OS.The bookmarklet found there performs some JavaScript to add a target=_blank tag to every link on the page. When clicked on an iPhone, the target bit forces a link to open in a new page, which in the iPhone's UI, is as close as you get to a tab.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070717101528804