My work background is kind of multipurpose computer nerd - I have some web coding background, some application coding background, some software & web testing background... I like to call myself a Swiss Army Nerd. Handy, multipurpose, but not a specialist.
I work for X, doing my Swiss Army Nerd routine in support of even nerdier, awesome ventures, which are in an area of particular interest to me. As in I'd happily turn from computer nerd to focus on this area of interest if I could only wrap my head around the advanced math involved. For now, my spot is solid, but in this economy, who knows how long that will be the case? We have, unfortunately, lost a number of good people, but the higher-ups are busting their patooties to bring in more work.
I have received a recruiting email from Y company, a solid company with a reputation as a great employer. Not related to the area of particular interest at all, but does draw upon some of my past work experience, would easily keep me interested and generally happy, and some of my past colleagues are employed by this company. One of them gave my name to one of Y's recruiters, who contacted me.
My question is, how to say, "I'm really really happy where I am and have no plans to move, but if the situation changes, your company is at the top of the list of places I'd like to go." I don't want to slam a door shut unnecessarily, but neither do I want to lead someone on that I am more immediately interested than I am.