Older thread, but since this question tends to pop up...
MSM720 has a lifetime SW warranty, so the new FW versions can be just downloaded from the device support site.
For other MSMs; a new device has one year SW warranty. So within the first year after purchase date new maintenance releases can be downloaded from My Networking portal after the device is registered. Maintenance release means that for a device running 5.7.x all 5.7-releases are free for one year. SW upgrade to a higher branch (i.e. from 5.7 to 6.x) requires a SW Care Pack.
With SW contract the FW versions are downloaded from a different location. Instructions are at http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c02665350.