Key signs that your innocuous gardening hobby is becoming something else:
You start saving seeds from your own open-pollinated plants in the hopes of producing a new, unusual hybrid. Like, say, squarish tomatoes.
You EAT the sort-of-square tomatoes.
(I really should have taken a graft of that plant. Think of the packaging savings!)
You start thinking of your garden plans in terms of years, not months.
You start regularly Tweeting about things like crop rotation.
You build something vaguely resembling a small science lab in your basement, for the purpose of starting entire flats of seeds indoors:
You start a new blog about gardening.