I said this might be a hurdle.

Turns out that DNC rules do prohibit cross-over participation. The state party has to comply with rule 2.E, which states:

"No person shall participate or vote in the nominating process for a Democratic presidential candidate who also participates in the nominating process of any other party for the corresponding elections."

So as far as the DNC is concerned, the objection of potentially disenfranchising Dems or indies who voted in the GOP primary is not valid. For the Obama campaign, this may well be a political hook on which to hang their argument about unfairness. But it's within the rules of the party.