i think it depends on how you see it.
the customer (i presume this is your job) wants the user to stay on the site as long as possible. if a site opens in a new window, the possibility that user will browse his site again, is much higher than if he has just the back button.
from the usability/information architecture point, the target="_blank" is rather bad imho.
just my 0.02 euro ;)
regards, michael -- niij <http://niij.org/>
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