What level of SOA maturity is required as a prerequisite for effective enterprise mashups?

None!  An SOA can be a great ENABLER for mashups but a good mashup platform can support lightweight 'virtual' services on data sources (which you use until these data sources are formally SOA-enabled).  We wrote a bit about this in SOA Magazine (http://www.soamag.com/I18/0508-1.asp).