I consider TDM as a REAL good thing in a Mac, but that have been discontinued as you can see here wikipedia.org:

the only Macs which do not provide for any kind of disk mode (supported or otherwise) are the original iMac and iBook series, and the MacBook Air and the late-2008 aluminium MacBook, all of which also lack a FireWire port.

Was it a wise decision? Many users did not think so: Apple users criticize lack of FireWire port on MacBook. I support the opinion at this time. Future will most certainly show new ways to gain the features of TDM, but i have not seen any yet ;-)