I've just uploaded a new ARC revision (rev 2009-03-04). In preparation of a Store Optimizer (which will improve the RDF store's scalability and performance), I slightly changed the underlying MySQL table structure. Please backup your data before you upgrade. Here's sample code, it's not too complicated:

/* old ARC version */
$store->createBackup($backup_path); // make sure the directory is write-enabled
// on success: $store->drop();
/* new ARC */
$store->query('LOAD <' . $backup_path . '>');

(Note: Store settings, if used, are not part of the dump and have to be manually copied over)

Main changes in this version:

• the (crappy) inferencer was removed, I'll add a rule-based system at some later stage
• the store indexes can be defined via a "store_indexes" config option now. Default: array('sp (s,p)', 'os (o,s)', 'po (p,o)'). The previous stores had an additional index 'spo (s,p,o)'.
• The store got an "extendColumns" method which changes the column types from MEDIUMINT to INT. You don't have to call this method explicitly, the tables will be auto-upgraded should your store reach ARC's previous 16M triples limit.
• There is a new "store_write_buffer" config option (default: 2500, changed from 5000). This option let's you set the batch size of triples written to the MySQL tables. In certain situations, esp.with shared hosts or large literal objects, the "5000" was too much and led to MySQL rejecting the queries.
• the toTurtle and toRDFXML methods (and associated methods in the
  Serializers) accept a 2nd "raw" parameter now, in case you don't want a full RDF document, but just the triples. (thx to Claudia Wagner for the suggestion)

If you have questions, just send them to the mailing list and I'll try to help.