This extension comes handy if you need to call a WebService during the test.
Also you can develop simple web service (python, .Net, ...) to do some 'utility work' - clean up a database, execute sql, etc. and use it in your tests.

attached file contains required js code (tested with selenium 0.8.0)

Examples of implemented commands:

storeValue
resetWebServiceParameters
addWebServiceParameter	user_name	Doe
addWebServiceParameter	pass	${password}
addWebServiceParameter	timeout	${timeout}
callWebService	http://somewebsite/webapp/_qa_/test_automation_svc.asmx/get_email	${result}
.. do something with ${result}

P.S. thanks to fellow MetaCommunications' developers for providing me with working code for the extension.