Hi there,
I started a discussion about this problem already in the LR4 forum. But as this is related not only to LR, but seems to be a general problem of ACR, I thought to add a link also here to attract also other people experienced with ACR, but maybe not so much with LR... esp. Adobe staff would be highly appreciated to jump in! 
http://forums.adobe.com/thread/1097304?tstart=0
Short version: Photos (RAW, of course) made with an Canon TS-E 24 II are rendered extremely pixelated both with LR4 and PS5.