Rescan doesn't show new LUNs on FC HBA...need to reboot?

On our ESX 4.1.0 boxes, we attached an unused FC HBA from each server to a new SAN and presented LUNS.

The HBA's always did show up in vCenter, but I expected that after doing a "Rescan All" the new LUNs would show up as devices on those HBAs, but they don't.

I'm told that "Rescan" never works for us, and the server needs to be rebooted.  Is that true?  Seems like a reboot shouldn't be necessary.

Thanks....