with one HBA the only failover you are going to get is at the service processor level on CX or at the FA level on DMX, your HBA has to be zoned to SPA and SPB on Clariion and/or to two different FAs on DMX. Any other failure such as switch/SFP/cable/HBA will bring your system down. You will need to use PowerPath ..unlicensed will work just fine.

As Dynamox stated, PowerPath supports multipathing through one HBA as long as you are aware that you have a Single-point-of-failure (SPOF) in your environment. If any of the HBA/Cable/SFP goes bad, you will lose access to storage down all paths.

Not to mention a reboot of the switch. Reboots occur after power failure (seen it happen for real) or an upgrade of the firmware of the switch from a very old version to any newer version (Cisco SAN-OS 2.x, McData 5.x (I think). AFAIK Brocade doesn't suffer from this at all, but I could be mistaken, since I've only seen 3800's and up.

If you are using Clariion then you definitely should do multi-pathing through your single HBA. Updates to Clariion Service Processors require reboots which would mean an outage to your environment.

If you are attached to a Symmetrix or DMX you could multi-path to multiple ports but other than hardware failure there is a lot less likelihood of a Symmetrix FA port becoming unavailable.

As has been mentioned by others the SAN switch or the host HBA experiencing an outage are probably much more likely failure events than the storage ports on either a Clariion or Symmetrix array so if at all possible if this is a critical environment you should push for dual HBAs.

If this is a test/non-critical environment I'd suggest only dual pathing if you are using Clariion.