As NT3RP told us that:
... we (Yahoo!) don't have a Finance API. It appears some have reverse engineered an API that they use to pull Finance data, but they are breaking our Terms of Service...
So I just thought of sharing this site with you:
http://josscrowcroft.github.com/open-exchange-rates/
[update: site has moved to - http://openexchangerates.org]
This site says:
No access fees, no rate limits, no ugly XML - just free, hourly updated exchange rates in JSON format
[update: Free for personal use, a bargain for your business.]
I hope I've helped and this is of some use to you (and others too). : )