Teams in the Cactus League are averaging 4,472 fans per game, down from 5,719 in 2008. It's still relatively early, of course; we're still not to the point where any schools are out for spring break, so we'd expect a healthy uptick in a few weeks. And remember this is the per-game average: with the additions of the Dodgers and Indians to the Cactus League this spring, Arizona will set a new overall attendance record.
We've not seen any similar numbers from the Grapefruit League, but anecdotally we can say things are down there as well. Boston and the Yankees used to sell out game consistently, but coming into spring only the Tampa Bay Rays had sold out every fixed seat on the spring schedule. Mets and Cardinals spring averages were reported as bring down 15 percent or so as well.