Where are the system spellchecking dictionaries?
The spelling dictionaries you are interested in appear to be located in the following location (checked on 10.8.4 and 10.6.8):

/System/Library/Services/AppleSpell.service/Contents/Resources/


The word lists are stored in this directory by language, so U.S. English is in the English.lproj folder.

However, these files are stored in a binary format that I haven't deciphered yet...