This is a known issue affecting a subset of Mac users.
Users who run Torchlight under Mac OS X 10.5.8 (or earlier) in combination with Intel GMA graphics devices may encounter a reproducible crash while in town. This issue does not affect Macs with nVidia or ATI graphics chipsets.
How to Resolve
The only known resolution for this issue is to update to Mac OS X 10.6 Snow Leopard.