another thought, and some corrections...
another possibility is that HP thinks this traditional monolithic high-end array market will shrink away to nothing over the next few years to be replaced by a combination of mid-range offerings and a "storage grid". I seem to recall some folks in HP talking about this a few years ago. In that scenario I could see HP continuuing its relationship with Hitachi right up until that market becomes unsustainable whilst delivering their own storage grid capabilities on commodity hardware.
and HP has no OEM relationship with HDS. Both HP and HDS OEM the USP-V from Hitachi Japan. Sun then OEM the OEM'd HDS version of the array. The point is that HP are not dependent on HDS for their XP business. Hitachi is the parent company of HDS.