[Don't you wish you could edit the subject line sometimes?]
I've a few times in my trip reports posted links to commmercial services that I used. I haven't been accused of spam yet.
Also important (I think) is that one always disclose potential conflicts of interest:
"They've got the best doughnuts in the world, but don't take my word for it, ask my uncle, who runs the place."
[ Edit: Edited at Oct 11, 2006 3:51 PM by pfeiffer ]