PRweb don't allow html but you can still put links in using their own code.
I've just done my first couple of press releases on PRweb - not sure if the money would have been better spent on links or something else at the mo' (did one really quickly to get it up that day and managed to knock my own page off G for duplicate content - duh).
Gonna try the free / cheaper ones next so I'll report back when I've got some results through - PRweb seems v expensive for what you get, but only time will tell.