After my black rubber thing fell out, instead of gluing it back on, I decided to try to find an alternative since it was worn down anyway. My friend has a new phone and his was about a millimeter above the joystick, and my had worn down to being level with the joystick. Anyway, I ended up cutting up one of the foam spacers that come with DVD and CD 50/100 packs. I used a 9-volt battery to make an impression and just cut it out. It looks a little ghetto as it started out white and is a little grubby gray now though. However, the nice thing is that it's hardly worn down at all in 8 months and is a little softer than the original -- and the wear that it does have actually has rounded it out a bit.
I was looking at other possible replacements and I think the buttons from this RCA remote would be perfect. I've compared my phone to it in the store and the size looks pretty close. It's fairly soft and I would think durable since it was meant to be pushed on a lot. I keep debating buying it but it's like $18 in the stores and that's a lot to pay for something to just end up cutting it up. But now that I look, there's a cheaper version online ($7-$8 ) so I might try it. Looks like you could get 11 replacement buttons out of it.
RCA RCR460
http://home.rca.com/rcahome/images/RCR460_hires.jpg