I confess to not staying up on this issue as it does not affect me but there may be some limitations on the usage of the 30-day "Tourist" visa exemption?
I think some changes were recently made re: land border crossings/entries, like only two per "calendar year"
http://www.thaivisa.com/forum/topic/...-more-15-days/
but do not think there is any current limit on international airport arrivals? (edited to add: there are some anecdotal, individual reports that after six (6) visa exempt entries via an international airport arrival over some period of less than 12 months, some IO's may start to ask questions about your reasons, probably trying to determine if you are "employed" in Thailand without the requisite documentation.)
Obviously a real tourist visa might be best for some, and most of these are still free (promotion extended again I think), if you get one at an official Thai Embassy (and not a consul). A
Re-Entry Permit can be obtained during an international air departure, for a fee, which will "preserve" the shelf-life of an existing entry for future use, regardless of type (exempt, real visa, etc.).