Linking to external sites is no problem at all.
OffTopic:
Bjørn, sorry. I once wrecked hundreds of posts on another site
unintentionally just by rearranging my viewing rights on Flickr.
How did that go?
I thought:
1) to many public pics on Flickr are confusing
2) I made all pictures private
3) I selected 500 public pictures from roughly 5000 uploaded ones
4) Result: I lost all of my favourites, all my pictures were reassigned a newly generated link, all of my posts with flickr pics were desintegrated
Soooo ... I guess if you want to keep the integrity of this site, even if people change providers, which real people sometimes do.... give this policy another thought or two...