In all fairness though, such topics posted in the general forum (where the moderators usually hover around) they do almost always get locked. For example:
http://forums.steampowered.com/forums/showthread.php?t=888152 Personally though, I do like to hear of sales all over, especially for services I may not usually use (hence why I added such to my site), but I tend not to post when I add to those sales here unless in reference to a GOG sale.
Actually looking at the rules again, even my thread about the Wishlist could be taken as a form of advertisement especially now the primary reason I posted it here has now been addressed directly by GOG.
Then again, consider too that GOG relies on word of mouth to attract new users, and some of those probably come via other DD services -- indeed I first heard of GOG via the Steam forums.
Meh. In fairly tired rambling state, I'm just trying to say that I don't think such topics should be a huge problem, but perhaps refrain from linking -- if someone is interested in the topic, I'm sure they'll find it via Google. That sort of rule works fairly well for some places at least.