I've never really thought of myself as a single system gamer. I started out with D&D 3.5, but I got into d20 Modern fairly quickly, and GURPS not too long after that. I was generally aware of the larger universe of roleplaying games, and had designs to try a number of them. I wasn't one of those people who never played anything but D&D.
Except that for the most part, I never did play anything that wasn't d20 based. My flirtation with GURPS crashed and burned in spectacular fashion, and I spent the next few years playing and running D&D, Arcana Evolved, d20 Modern, and Star Wars d20. Since then starting college, I've played in a short GURPS campaign and ran a session of Feng Shui, but the only serious campaign I've ran was 4th Edition D&D.
And yet, somehow, I've never identified as a D&D gamer, or a d20 gamer. It wasn't even that I wanted to play other games, but always ended up playing d20 because that was all my group would play. The people I play with have always been fairly open to new games. (Upon seeing the new World of Darkness books I had in my backpack, they each picked a splat and started making characters for fun.) I just always ended up playing d20 games.
I only really noticed this when I started writing about the new World of Darkness. Lots of responses, lots of advice, lots of people who are very much not D&D players -- rather than D&D players who dabble in other systems, like me and a lot of other bloggers out there. Or, as the case may be, players of no particular system who nevertheless end up playing D&D and D&D derived systems most of the time.
The way things are looking, though, I won't be playing anything d20 for a while, and D&D is only a little less unlikely. Several of my prime candidates for players in a campaign next semester don't like d20 at all, and I've been in the mood to experiment with another system for a while. I might still end up running Swords & Wizardry, which is close enough to being D&D to count, but Traveller, Mage, and Vampire are all very much in the running. My home group hasn't discussed next summer much, but most of our recent gaming has involved experimenting with new systems. I might not be a d20 player again for a while.