RAID is bad for Squid. Squid's massive numbers of small files is the worst possible workload for any striped RAID configuration. With a striped configuration, every file gets split into some size chunks for writing to disk. Squid's average object size is only 8-13kbytes. So generally, instead of having the latency of a single drive to wait for each object to be loaded from disk, you end up with the latency of all of your disks for every object.
RAID is good for large files, and truly bad for small files.
Let Squid do the dividing amongst your disks. Yes, the Squid we used at the cacheoff has a new storeio module called butterfly, which was written by one of the ReiserFS guys. We also had a modified ReiserFS version that provided the raw API for butterfly to talk to. But it obviously does make a big difference in performance. The work the ReiserFS guys are doing now is slowly working it's way into the official Squid.