Wow. Words fail the awesomeness of this app.
I can feel this replacing several of the tools I currently use as a DM.
As a DM I wanted to make the following suggestions/feature requests.
In the future, you might consider an option on "how" you would like to sign in to Power2ool. Perhaps a DM view and a Player view. If these workspaces were able to sync to each other, a DM view could have tabs for each players workspace, in addition to their own. When in a tab for a player, you could let the DM be able to add notes/items/whatever that can be toggled as invisible to the player. This way the DM can organize notes about a character, or relay exactly what it was they were able to steal from their fellow player at the right moment.
Implementing a "Character Sheet" widget/gadget would really help a poor DM keep up with the abilities/items/stats of others on the fly. It would be nice to have a collapsible (minimize/maximize) widget with stats for a player. Nice and easy way to check things like passive perception/insight, base stats, those sort of things. Again, letting that info propagate across from a Player to a DM would be awesome.
Imagine something like "stacks" in osx, but they would be a stack of "cards" with only the top titles peeking out until you click them. Then they expand to show all the cards. A DM could use this to have stacks of power cards for each player. Then when they needed to quick reference a player's ability (I don't know many GM's with all of these power memorized...) they can click on the stack. When they click off the stack, the cards all flop back into a nice little space.
One last idea. A print option to create an export file that can be printed to 4X3 blank note cards. One "virtual" card at a time. A quick way to make consistent official and home-brew item cards would be slick. Or imagine a nice little set of monster stat cards...printed right from power2ool.
My obsession with printed note cards can best be understood by viewing this blog entry (not mine).
The Creak of Boots: ILLUSTRATED WILDLIFE TREASURY
Anyways, please keep up the amazing work. Haven't seen something this promising for 4e since before Wotc shut down Masterplan compendium integration.