Or does it mean that the Maliki government is cracking down on the Sadr militia but not the Badr militia?

Excellent point, Dave. I've been digging for news of Badr arrests and, well, give that coupe a spin yourself.

The Badr Brigades are more closely held by Iran than Muqi's boys.

So it is of much significant that the report of the 400 arrested (make what you will of that number) Mahdi Army members were a.) over the past couple months and (more significantly) b.) in southern Iraq, the natural home of Badr, vice Mahdi's Baghdad summer home.

If they're in US custody, I'd take it as a sign that Maliki really is moving against Sadr. If they're in Iraqi custody, I'd suspect that it's a sham, just because it's easy for an "imprisoned" capo to keep running his crew until such time as someone decides it's safe to kick him loose.

...

U.S. and Iraqi forces arrested a top aide to radical Shiite cleric Muqtada al-Sadr on Friday in Bagh...

Now, this strikes me as good news on a number of levels. It would seem to provide additional evidence that the al-Maliki government is cracking down on the militias, even those to who he’s beholden.

Or does it mean that the Maliki government is cracking down on the Sadr militia but not the Badr militia?

However, like you, I have to wonder as to the underlying politics.