I was upgrading my home system remotely and it didn't come back up after the second reboot. Unfortunately, it's the only host available remotely, and I won't be able to see why until later tonight when I have physical access.
I'm hoping its just something from ports which isn't happy. I wasn't prompted to rebuild, though (the first time I can remember that happening).
Yeah, after the second (in your case 3rd) reboot, I no longer have ssh access and not enough access to the rest of the network to tell why. I was able to successfully reboot into the 9.1 kernel after the first upgrade/install. It was only the second that didn't work.
You don't actually have to unless you're moving from major version to major version (i.e. FreeBSD 8.x to 9.1 in this case), though it's never a bad idea to do so.
The FreeBSD ABI doesn't change with minor version releases so the majority of ports should work through all of the 9.x releases without needing to be upgraded. The big exception is ports that work closely or otherwise depend on kernel functions, those may changes and therefore those ports will need to be upgraded in order to keep working.
I'm hoping its just something from ports which isn't happy. I wasn't prompted to rebuild, though (the first time I can remember that happening).