REST -- we are working on it. (1. Nov 2011) Check developer mailing list and latest code. Other language independent API--- we have it for the new code in the form of discoverable SQL interfaces, and are working on rewriting the rest with that in mind
Depends what your online store produces. We're currently ( 1st Nov 2011) working on creating web service API's but freelock (a frequent visitor of the IRC channel) is already doing it. He doesn't mind sharing his code.
Web service API's discussion is ongoing on the development mailing list. Join and contribute
From the users mailinglist (2011.05.22): Nigel Titley has an operational bridge working between Oscommerce and LedgerSMB. He is happy to throw into the pot, especially once the Ledgersmb RESTful interface comes out of the woodwork. It transfers new orders, new parts and new customers across from OSCommerce to LedgerSMB, handling multiple currencies and it updates the OSCommerce inventory from LedgerSMB. It runs as a regular cron job. http://ledgersmb-oscom.svn.sourceforge.net/viewvc/ledgersmb-oscom/trunk/...