Funny you should mention that Jamie! In my testing of the new Web Service linking features in Service Desk 7.7.1 I use a currency conversion web service as a simple example.
Here is the web service I have used successfully: http://www.webservicex.net/CurrencyConvertor.asmx. You can call it using the new feature and just give it two currency codes (for example GBP and USD) and it gives you the exchange rate. You can save that to a Decimal attribute and then use that however you want, including in a Before Save calculation with that attribute as a dependency.
I have it set up at the moment so let me know if you'd like more info. I might even do a doc/video as a general example of using the new feature.
Thanks
Stu