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I'm not sure, I haven't thought about it in a while. I don't feel like it's solved, but it does feel like a point of diminishing returns. (Well, for me and Skilgannon. The rest of you have work to do. :-P) Then again, that feeling also precedes most breakthroughs.
I do think any general improvements to movement would still fall under the banner of "Wave Surfing". But I certainly think there are improvements. Heck, maybe a lot of them - for all the bots in the rumble and all the activity on the wiki, there's actually very few people in the world that have reached the upper echelons of Robocode and kept trying to improve on the state of the art. I'm sure if 5,000 PhD students spent a few years building Robocode bots, they'd do pretty well. :-)
But I think there's a lot more fertile ground hidden by our metrics, too. We focus pretty strongly on APS. Only a few top bots (eg Shadow, DrussGT, Diamond/Dookious) have any kind of evolved strategy for fighting other strong bots. I'd say we've mostly side-stepped the arms race between adaptive movements and anti-adaptive targeting, which could be pretty interesting.