Lightmapping 'block' bug, so annoying...

So, I think this has happened on numerous occasions when light-mapping... Beast comes up with good looking lightmaps, but leaves these 'block' artifacts. See the picture below if you want to understand better:

Gross, eh? It seems to show up on white surfaces such as the one below, it probably shows up on other surfaces, but is just harder to see. Anyone got any ideas on how to fix it? Am I the only one this happens to? Do I need to file a bug report to Unity? (P.S: Mac OS 10.6.8 ATI Radeon HD 4670, if that helps) Thank you for your help!

-Stopsecret