First of all, you MUST include QuartzCore header, else meet some interesting error notes:
#import <QuartzCore/QuartzCore.h>Then add this code after your cell initialization routine, usually at cellForRowAtIndexPath:
CALayer *layer = cell.layer;Like many things in iOS SDK, you get to a certain point with relative ease - and this is about it. If you want to have more control over your "bubble button" you either dig deep into Quartz sample code or implement something which looks same in a completely different way.
layer.borderColor = [[UIColor blueColor] CGColor];
layer.borderWidth = 3.0f;
layer.cornerRadius = 20.0f;
For example I replaced this (got a bit further with carefully positioned UIViews inside UITableViewCell) with hardcoded bitmaps and scaling them nicely.