Upon analysing the logs for my site I noticed an alarmingly large number of 500 Server Errors against rss.asmx.
It seems the problem is caused as the method names (the bit after the last slash) are case sensitive. I.e. rss.asmx/rss is not the same as rss.asmx/RSS.
There is one rather interesting twist to this though. If someone usees the web service with the correct case, the response is cached. The wrong case will then work until the cash is cleared.
If anyone knows a way to make web service names case insensitive, please leave a comment, until then, I am left with re-writing my RSS feed as an aspx page.