They were originally planning a 'first 100 buyer's' list to be displayed at the end credits. It was 'incomplete' or something like that, you can get it by downloading an older version of Minecraft. I think it would be nice to get a cape (Even if I wouldn't get one, joined around 1.3). It's a fairly basic design, and isn't something extremely OP, like 1,000 diamonds in each starter chest. I guess it would be cool.
It would be nice, but honestly, I don't care if they do something like that. I doubt they'll do something, though, since it's been long enough from any notable milestone that if they wanted some cosmetic thing for early players, they would have been added by now.
I bought the game back in 2010 for 15 bucks I think. I saved some money and got to play it longer so I guess that's enough reward for me. I personally don't care about a cape, but if I got one (assuming Mojang even has those records) I wouldn't mind.
The Meaning of Life, the Universe, and Everything.
They've thanked early adopters [5 Dec 2010 ] by continuing to develop the game... sure there's been some speedbumps but for the most part it's been a fun ride. Jury is still out on Minceraftsoft though.