crj705
There is no limit on back to back visas, but that applies to real visas that you have to get at the Thai embassy abroad. Visa free entries, which I think you are talking about, are limited to 90 days every 6 months. That was the change in 2008 along with visa free entry at land crossings cut from 30 to 15 days.
Sorry to harp on but even the 15/30 day visas upon arrival are (currently) not limited to 90 days every 6 months. This regulation was relaxed when the 15 days across a land border was brought in.