Yes, they're combined. For a specific example:
Suppose the play count is 7 when you sync. It's now 7 on both devices. You disconnect the iPod and carry it with you.
While you're on the road, you listen to the track once. The play count on the iPod is now 8.
Meanwhile, a family member listens to it twice at home. The play count in iTunes is now 9.
You come home and sync. The play count in both places is now 10. The iPod has remembered that it played it once since the last sync, and adds that 1 to iTunes' current count. The "Last Played" timestamp now reflects the last time it was played, no matter which device that happened on.
All of that is the way I hoped it would be, but I didn't want to reply until I had a chance to test. I knew that playing on the iPod updated the information on iTunes, but for all I knew (until today) it was combining the counts by simply keeping whichever was larger. Leave it to Apple to do it the right way, rather than the easy way.
If you share your music library across a local network, and play it from another machine, the play count does not change. Shared access does not imply write access.