First two digits refer to the factory...
I believe all start with 87 = China or (replacements) 5K = China
Next digit = year... so if you have 8 = 2008... 9 = 2009
next 2 are the production week... so 30 = 30th week of 2009
then everything after that I believe is the unique identifier....
This is all the info I've been able to find... if anyone else has any info that'd be cool!