Let's just say that I'm not the biggest fan of the ObjectDataSource. At netTiers, we had to write a custom control, the EntityDataSource to make up for all of the ObjectDataSource shortcomings.
I was baffled when I discovered this, but one of the most annoying things about the ObjectDataSource was when using a ControlParameter to specify a parameter from an existing control there was no clear way of using an existing page level property. So I very hackishly used a server side hidden control so that the ObjectDataSource could read that value from the form.
Well, a team member on .netTiers, Bobby Diaz, discovered this nifty little nugget of gold.
<asp:ControlParameter Name="MyMethodParameter" ControlID="__Page" PropertyName="MyPageProperty" ConvertEmptyStringToNull="false" />
I've always seen that in the trace output, but I never put 2 + 2 together.