From a couple of web searches it sounds like they're on MySQL. Targeting a database that has working transactions has to be less work than implementing transactions yourself from scratch, which it seems everyone stuck on MySQL eventually has to do (that's what they're describing, and I know we've done similar things).

Depends what MySQL table types they are using. I think MyISAM has no transactional ability.

I'd argue any database on modern hardware where you can't do a single transaction on a few hundred rows without noticeably affecting performance does not have "working transactions".

Rails has always been very MySQL-centric, probably because 37S & DHH use it for Basecamp. Perhaps now that PostgreSQL v9.0 has built in master/slave replication they will finally make the switch.