We have not gone through a full round of regression testing, however, with NPM we did perform some high level testing against the RC for vSphere 5 to make sure nothing broke and everything appeared to be working properly. If you find anything let us know, but should be ok.
At VMworld last week, I was told my a few SW product reps that yes, it should work, since v5 has monitoring API that are backwards compatible with v4. Nothing you can see/do in v4 should be lost if you upgrade hosts to v5.
However, since v5 has some new tricks, these won't be available through NPM until a future NPM upgrade.
I can't confirm this yet, but I'll be upgrading my hosts shortly, so... I have my fingers crossed.
