Today Xero released some extra functionality to their API including JSON responses, manual journal entry, items and PDF invoices.
Of great interest to me is the ability to download PDF invoices directly from Xero which are formatted according to the chosen branding theme. This functionality will greatly decrease the time to market of my new application (still in stealth) by a few weeks and has me very excited.
My ruby library (Xeroizer) will be updated to work with these new features within the next week or so. Keep a watch on the project to keep up-to-date as changes are made and feel free to submit pull-requests for desired functionality. I usually respond within 24 hours.