[Sigia-l] Usability of launching new browser window?

[lydia]

I thought I remembered this as a controversial topic, but now I can't seem 
to find any useful recommendations. The question is about whether to have 
external links launch a new browser window, so that the user still has a 
window open to your site. I've never favored this approach myself, but it 
has been suggested that I implement it for a client site. I'm looking for 
guidelines about the advisability of this practice, in terms of user 
experience. Do detractors consider this an accessibility issue, or just an 
annoyance? Can anyone point me in the right direction?
Thanks.

-lydia