(I edited my original post to correct my earlier information that was based on my current understanding from discussions with HP Support.)
After discussing extensively with HP Support, they will support either their pre-built ESXi 5.1 images, or creation of your own custom ESXi 5.1 image, so long as you use a driver/utility set they have tested with that version of the hypervisor. This is true even if you have hardware-only support with HP. I also looked more closely at what drivers HP included in their custom ESXi 5.0 and ESXi 5.1 images across June 2012 - October 2012. I noticed under ESXi 5.1, HP has opted to use the VMware certified lpfc and be2net drivers needed for HP FlexFabric controllers in the blade hardware.
However, for other drivers, HP is recommended to override VMware certified drivers for ESXi 5.1. So in those cases, the questions discussed in the following VMware blog article come into play, and leave the engineer to decide whether to use VMware or HP selected drivers when there's a conflict:
http://blogs.vmware.com/tam/2012/08/a-couple-questions-on-vmware-vsphere-5-image-customizations.html
I'm not sure which route you're going. I was originally on the fence but I'm leaning towards picking HP drivers over VMware when there's a conflict. Past experience has shown benefits to keeping the drivers and firmware in line with what HP documents. However per the blog post above, when getting support from VMware, they may ask you to reinstall their certified drivers over what HP documents you should use to match against the firmware. So if you have your support split between the two vendors, if there's ever a driver/firmware conflict, you're stuck between a rock and a hard place.
Not exactly related to your original question, but if you end up opening support tickets this is something to keep in mind.