Looks pretty good. I assume most (if not all) of the graphics are placeholders for now, so I won't comment on those.
Firstly, you might want to improve your text rendering - you're rendering fixed-width, but the Pokémon fonts are variable-width. It's a small change, but variable-width fonts usually look more professional.
Secondly, the player and opponent HP bars have different sizes, which looks strange.
Thirdly, you might want to make the move selection buttons bigger, so you can include the move's PP and type (does the colour of the button represent type?). This will also give you more space for the party Pokémon list and items list.
And finally, you might want to display the back sprite zoomed even further (or the front sprite at normal size) to get some perspective.
Also, good idea with the tabs. I'm also using tabs for the battle menu (independently of this game; I got the idea from various Pokémon battle simulators such as Pokémon Online); it's (in my opinion) a lot more convenient than a separate screen for each type of battle command.