You can find more than enough code samples here:
http://msdn.microsoft.com/en-us/library/ms742859.aspx[
^].
This CodeProject article can also be useful:
WPF Drag-and-Drop Smorgasbord[
^].
The key is: it's not important what is the source of dragged data, Excel or not, so don't pay attention for how it's done in the samples. All you need is the compatible data type from the provider. It looks like the problem of data types is explained in the article referenced above.
—SA