DanM - you can set up a poor mans load balance trick. The following info is from a Client lpar that is getting its disks thru 2 vio servers, each vio server has 2 HBAs, feeding back to 2 switches and on CX500. All we are really doing is spreading the load across the VIO hba's and efectivly (sp) spreading across the 2 SPs.
*** please test - I have this config, (# of paths vary), set up on 6 lpars - NMON shows a pretty good spread at the vio level.

Do something like this on the client lpars;
338 chpath -l hdisk0 -p vscsi1 -a priority=10
339 chpath -l hdisk1 -p vscsi0 -a priority=10
340 chpath -l hdisk2 -p vscsi1 -a priority=10
341 chpath -l hdisk3 -p vscsi0 -a priority=10
342 chpath -l hdisk4 -p vscsi1 -a priority=10
343 chpath -l hdisk5 -p vscsi0 -a priority=10
344 chpath -l hdisk6 -p vscsi1 -a priority=10
345 chpath -l hdisk7 -p vscsi0 -a priority=10
346 chpath -l hdisk8 -p vscsi1 -a priority=10
347 chpath -l hdisk9 -p vscsi0 -a priority=10
348 chpath -l hdisk10 -p vscsi1 -a priority=10