TPM points to this essay, in which FindLaw columnist Vikram Amar argues that the Wisconsin statute directing the governor to keep Craig Thomas's Senate seat in GOP hands is quite possibly unconstitutional.

There is a very strong textual argument that the Seventeenth Amendment prevents the Wyoming legislature from dictating the Governor's choices in making a temporary appointment: The Amendment's language differentiates between a state "legislature" and a state "executive" authority, and allows a state legislature not to make or constrain any temporary appointments itself, but rather only to "empower the [state] executive to make [the] appointment."

The essay's a bit boring (in the grand tradition of law professors), but it makes a pretty compelling point.