Having external links open internally isn't what I'd consider fatal, so you may choose to live with it failing for Opera users.
(It should be noted that Opera users are generally more savvy/discerning than average when it comes to browsing and are probably more inclined to have popups disabled. This means that a disproportionate percentage of them will not be able to access the new window target anyway.
Another consideration is that, being more savvy, they are probably better versed in using modifier keys to force links to open in new windows/tabs and would prefer to make the choice themselves.)
There are alternative methods, each with their own level of 'gracefullness'.
If you change your mind about the above script, let me know and I'll offer up one of the alternatives. One in particular is XHTML Strict compliant (i.e. no
target attributes allowed) and degrades well for javascript 'differently-abled'* browsers).
*
