How to display a base64 image within a UIImageView?

You don't have to encode it. Simply make a NSUrl, it knows the "data:"-url.

NSURL *url = [NSURL URLWithString:base64String];    
NSData *imageData = [NSData dataWithContentsOfURL:url];
UIImage *ret = [UIImage imageWithData:imageData];

As mentioned in the comments, you have to make sure that you prepend your data with data:image/png;base64, or else your base64 data is useless.

Very old question, but as of iOS7 there is a new, much easier way to do so, hence I'm writing it here so future readers can use this.

NSData* data = [[NSData alloc] initWithBase64EncodedString:base64String options:0];
UIImage* image = [UIImage imageWithData:data];

Very easy to use, and will not hit the 2048 byte size limit of a URL.