GTM always asserts while testing UIToolbar using 
GTMAssertObjectImageEqualToImageNamed() macro if toolbar style is 
UIBarStyleBlackTranslucent and it contains a button with style 
UIBarButtonItemStyleBordered.

Please observe images produced: failed and diff in attached archive.

Issue doesn't reproduced if to change   
CGImageRef imageRep = [self gtm_unitTestImage];
to
UIImage *image = [UIImage imageWithData:[self gtm_imageRepresentation]];
CGImageRef imageRep = image.CGImage;
in 
- (BOOL)gtm_compareWithImageAt:(NSString*)path diffImage:(CGImageRef*)diff

It would help us a lot if you could resolve the issue inside GTM...

Dev configuration: XCode 3.2.4, Mac OS X 10.6.4, iPhone SDK 4.1