I have found a sample of using MEF and Asp.Net Web Forms applications
In my opinion it is awesome but it requires re-inherit all usercontrols and controls to allow them importing parts.
My implementation is based on the given one (use scoped and global containers) but does not require to re-inherit controls and additionaly supports importing into masterpages, httphandlers and httpmodules.
You can find a detailed article and full source code at
Feel free to ask me any question about the implementation.