Lest the public be confused, the TEMPDB files of production SQL servers should not be placed on the same partition with the rest of the data / log files, or even on other partitions, per se. The key word is, in fact, not partitions, but rather SPINDLES. Having the TEMPDB file on some other partition, but still on the same set of disk spindles will cause almost the same performance issue as having it placed on the same partition.
It is a different story on a SAN though, and I generally ignore the spindle considerations, except on special cases. Different SAN vendors have their own "best-practices" for SQL on SAN, and this really depends on the underlying architecture of their file system. My experience is with the NetApp SAN, and allocating specific spindles (hence, partitions) for a particular db is not the general practice.