I suspect that Nokia initially hid the multi window abilities of the browser because early 3rd edition phones, like the N91 and N80, had insufficient RAM for the browser to load many sites without an "out of memory" error. But with the N95-3 I often have WebKit, the gMail application, Media Player, Yahoo Go and Opera Mini open simultaneously and I've never seen an out of memory message. Surely, having a few tabs open in the browser wouldn't hurt.
So I built a little page at that has this link on it:
<a href="tab.htm" target="_blank">Click to Open A New Window</a>
and saved it as tab.htm on wapreview.com. When I load the page and click the link it opens another copy of the same page in a new window. Not very useful by itself but once you have a few windows open, you can press the "5" key to bring up a nifty window menu with tabs (see image above). The left or right keys navigate between tabs. You can cycle through the windows and open a different site in each one by pressing "1" to choose a bookmark or "9" to key in a url.
To fill up the page and make it even more useful, I added a list of WebKit's shortcut keys, which aren't in the browser's help file, to tab.htm. If you want to try it, go to wapreview.com/tab.htm. You can bookmark it and run it from there but for faster response and to remove any dependencies on wapreview.com, I recommend saving a copy on your phone using Options > Tools > Save Page. Then you can open it from Bookmarks > Saved Pages.
I've found opening multiple pages pretty handy. I had no trouble simultaneously opening the full web versions of Bloglines, Yahoo Mail, gMail, Howard Forums and this blog in separate windows. There's a hard limit of 5 open windows in WebKit, I get "Unable to open. Maximum number of pop-up windows already open" if try to open a sixth window.