It's all well and good starting a contest up and posting on webmaster sites like these the details of your contest. However, I need my contest to appeal to people interested in discussing poker. I have tried posting on poker-related forums but I am not allowed to as I am in direct competition.

Blog commenting is the best I can think of but how can I do that in a non-spammy way?

It's easy.. Just make a relevant comment to the blog and have your "advertisement" ready to paste. press ctrl+V when your done with a relevant comment and then you can usually get approved.

Also, do this:

at google search you should search the following exactly how it is: your blogging search critera inurl:"blogspot.com" "post a comment"

This will bring up all matching blogspot blogs that accept comments. Start from the top and bookmark where you left off.