GBA games can use the touchscreen if they are programmed with touchscreen-capable gameplay. Games in the GBA slot are cabable of utilizing everything that games in the DS slot can utilize, it just depends on the software. So if someone wanted to, they could make a GBA game that also has optional features for players using a DS like a touchscreen control option, a control scheme using four face buttons, or support for wireless multiplayer on the DS's frequency and for more than just 4 players.
Of course, I'm not sure about what Nintendo would feel about this as it may alienate GBA users...Why would you want to play such-and-such on GBA when it's so much cooler to play it on DS? But then again, if you bought Perfect Dark and didn't have the N64 upgraded RAM, then you were screwed. And who's going to stop developers from putting in extra features for DS owners? Not everyone has a GBA-GCN cable but there are plenty of games with extra features just for those people.
By the way, there is a new Pokemon rpg in the works for DS. They are called Pokemon Diamond and Pearl. Thought you'd like to know since no one here seems to know yet...