There are some issues to consider here.
1) You could be seen to be diluting another's trademark. Dilution is seen as the "death of a thousand cuts", whereby one small encroachment repeated many times over can harm another's effective trademark. However, I consider this only to be a realistic argument when you use a trademark in the hyperlink itself. As Chris' link shows, there are yet to be any cases concluded on the matter.
2) Passing Off: A subsidiary of trademarks, this is where you attempt to pass of your goods (here a website) as that of a competitor. It's perhaps unlikely if you're linking to their genuine site.
3) Reverse Passing Off: This is the biggie. It's where you hold out your goods as belonging to/being part of a competitor's when in fact they are not. There is an argument that by linking their page to yours, that's what you're doing.
Currently, there is no legal barrier stopping you, only the threat of action from the other party. You should include a disclaimer that the link is one of an external site that is not under your control (this will also indemnify you against inappropriate content). If I were acting for you, I would argue that you are not using the link "in the course of trade", which is necessary for a trademark infringement. There is no potential for copyright infringement here, for as you say, you are not copying it.
I know I've not backed up my answer legally, but I do have a postgrad qualification in IP and have experience in the area. To put this into perspective, see how the BBC do it on a daily basis. Never a hint on an infringement here!:
http://news.bbc.co.uk...transfers/8685915.stm