Hey edmc,
There's nothing wrong with using StumbleUpon to recommend each others sites for back-links and traffic generation as long as you are recommending high quality sites to fellow stumblers.
That's not spamming....... that's creative marketing.
http://www.stumbleupon.com/tag/poker/
has PR 4 and probably gets a descent amount of page views.
Search engine traffic is still king....... but more and more web surfers are using Web 2.0 sites like StumbleUpon to find the information they seek about their favorite topics.
It's the wave of the future and
every internet marketer should be paying very close attention to this new development in how people use their web surfing time.
Obviously you want your affiliate links in front of as many potential customers as possible.