You can do a search on "tablets", and that returns many more apps than are listed in their "tablet showcase". Variations are to search for "honeycomb" and for "xoom". You should find lots of hits, but you unfortunately have to weed through the results, which won't be easy.
(For example, there's a bug in Market where it won't preserve your position when scrolling through dozens of rows of search results and you touch an app name to look at the details. You may have been at row 40 when you touch that app, but when you hit the Back button you are not taken back to row 40, you are at row 1 again and have to scroll all over to figure out where you left off. I've reported this to Google and received acknowledgment they are looking at it.)