Where can I find XML-formatted currency exchange rates that get
updated daily (or on some relatively frequent, regular basis)?  Must
be a free service.

mrfrog,

Here are some free XML-formatted currency services on the web:

  European Central Bank:
  http://www.ecb.int/
  Euro-based daily updated XML list of 28 country currency rates

    Current: 
      http://www.ecb.int/stats/eurofxref/eurofxref-daily.xml
    Historical daily rates back to 1999:
      http://www.ecb.int/stats/eurofxref/eurofxref-hist.xml

  SOAP Web Client
  http://www.soapclient.com/index.html
  Shows how to send SOAP message to use web service that computes
current exchange rate between any two currencies, and returns the
result as an XML message

    http://www.soapclient.com/soapclient?fn=soapform&template=/clientform.html&soaptemplate=/soapresult.html&soapwsdl=http://www.xmethods.net/sd/2001/CurrencyExchangeService.wsdl

  UDDI.org
  http://www.uddi.org/find.html
  A site that categorizes all publicly available web services.  A
direct link to results is not possible, but a search for "currency"
reveals 13 currency related XML web services available.

Google search terms used:
  XML currency feed
  XML free currency exchange rates RSS
  XML web services currency exchange rates

Great!  That does the trick nicely.  Thank you very much!