Apparently you can install hpacucli in ESXi very easily, and whats even better is that you can install it directly on a datastore and use the install on any host that has access to that datastore. No need to reinstall on each and every host in your cluster. No need to reboot to configure your disks.
Anyway here is the link to the how to, its obviously written by a Windows Guy, but it works easy enough.
Update: Apparently HP now provides an official version of hpacucli for ESXi 5 as part of HP ESXi Utilities Offline Bundle for VMware ESXi 5.0.
Also note that I found that my HP servers with the P400 card did not detect the new logical drives, even after a rescan. Apparently a reboot is required. However my HP boxes with the P410 card did detect the new logical drives after a rescan and no reboot was required.