I received one from Dennis, but didn't consider it spam. You can always respond back and say "no thanks" if you're not willing to participate at the moment. Like others have indicated, Dennis has been a great contributor to these forums - and has done so for quite a long period of time.
However, if I had received this message from a newbie (someone with less than one month in these forums), I would have considered it differently.
Think of Dennis' message as a public service broadcast - or a recruiting effort. It's not like he promised to make you a millionaire - or some other silly topic like the actual spam messages you receive via E-mail.