Some would say this doesn't matter that much, but I can empathize - I'm kind of an info-nut myself, I like to know what's going on with my device, and if I had an external battery, I'd want to know the remaining power figures. Well, Transformer owners, there's finally a solution to this conundrum. Two solutions, actually.
Battery And Dock Widget by beeone42First, we have an app by XDA member beeone42 - it's a simple visualization widget that shows the status of both the tablet and dock batteries on your Transformer, as you can see in this itty-bitty screenshot:
app here. (Note: reboot your Transformer if the widget doesn't appear in your widgets list) Be sure to check the thread to see if there's an updated version, though.
Transformer Battery Widget by Highland3rThe second widget is for people that like their UIs to be a bit more text oriented, as you can see in this screen:
here. You can get the black version here. And here's the link to the XDA thread.
Both of these apps have been tested on rooted and un-rooted Transformers running the 3.1 update, so if you're still on 3.0.1, they may not work properly.