What I mean by "optimized for honeycomb" is either:
a) The developer has updated/created the app using the new SDK and design scheme
-and/or-
b) The developer has specifically updated/created the app to make use of the larger screen space, dual cores, and gpu that this new line of tablets will use. This extends to recognizing and acknowledging that landscape mode is the more natural position for tabs these large and rendering properly as such.
There's some grey areas because there are tons of apps that work really well on the Xoom just because unlike iOS, android apps are made to scale well out of the box thanks to the sheer variety of devices they must support.
Pulse was made specifically for honeycomb and it's on the list. Adobe Reader isn't downloadable from the market this morning for unknown reasons, but I will test it as soon as it works.