I just noticed that running a list=usercontribs query returns a totally wrong value for query-continue. at the moment, you get:
<query-continue> <usercontribs ucstart="timestamp" /> </query-continue>
when you really ought to be getting:
<query-continue> <usercontribs uccontinue="timestamp" /> </query-continue>
the timestamp value might be the correct value (I'll try that and see) but the parameter name is wrong. it even says this in the docs, incidentally...