Hi, yes you can use SSD's with no issues.
I currently run 4 x Corsair Force 256MB 6G SATA SSD's in the std backplane.
There are a couple of caveats though:
1: Unless you upgrade from the default P400 to a P410 controller you will be stuck at 1.5G SATA speeds per drive, which in actually operation isnt much of a handicap as it's inlikely you will generate random bus traffic requiring more than this per drive. If you do feel a bottleneck then a P410 will offer you 3G sata transfer rates which again should be fine for most needs.
2: If you wish to use SAS drives also then you will have to split the cage between controllers as the limitations present in firmware prevent a SATA array from taknig precedence over a SAS array in the P400 firmware thus it will always try to boot from the SAS array and never the SATA array. I got round this by purchasing a 2nd P400 controller for £20 on eBay and connecting the 2nd set of bays in the drive cage to this controller and runnnig my SAS drives from that & having the P400 with the SATA SSD's as my boot controller.
You may get replies from HP support folk who will religously point you to quickspecs saying it's not supported, this DOES NOT mean it doesnt work it just means HP will not give you support on this, which is a fairly usless response from them as you are unlikely to have a support package for a 6 year old server in the 1st place.
hope this helps.