Free tool that could help sell mobile sites
http://code.google.com/apis/analytics/docs/gdata/gdataExplorer.html?dimensions=ga%253AoperatingSystem&metrics=ga%253Avisitors&segment=dynamic%253A%253Aga%253AisMobile%253D%253DYes&sort=-ga%253Avisitors&start-date=2010-01-01&end-date=2010-12-31&max-results=50
Just be aware that it's a two-edged sword. It will show if they need a mobile site, but it will also show if they have few mobile visitors.
Since you should never try to sell something that a client doesn't need, that should be OK.
Enjoy!