You can, in Firefox it works, add the attribute target="_newtab" to the anchor to force the opening of a new tab.
<a href="some url" target="_newtab">content of the anchor</a>
In javascript you can use
window.open('page.html','_newtab');
Said that, I partially agree with Sam. You shouldn't force user to open new pages or new tab without showing them a hint on what is going to happen before they click on the link.
Let me know if it works on other browser too (I don't have a chance to try it on other browser than Firefox at the moment).
Edit: added reference for ie7
Maybe this link can be useful
http://social.msdn.microsoft.com/forums/en-US/ieextensiondevelopment/thread/951b04e4-db0d-4789-ac51-82599dc60405/
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