Well lookie here, a new PSP firmware has conspicuously appeared on Network Update. As you'd expect, firmware 5.51 is a minor revision, with the supplied changelog vaguely mentioning of improvements in "system software stability" during use of certain features.
Not to sound cynical, as stability enhancements are always important, but in the end this amounts to changes we will likely never notice. That makes this firmware version bump a rather uneventful one. Here