That sounds unlikely. They spent quite a bit of time and money on this DLC, they're not going to see whether it 'succeeds' on 360 and just throw it away otherwise. It will be released. They've spent money making it, they want to make that money back. Limiting themselves to selling it on one platform for no reason whatsoever (since the exclusivity deal will be over) makes no sense, because they're guaranteed to make more money by bringing it out on PC and PS3 as well.
They'll probably release it as soon as they can after the exclusivity deal, it's in their own best interest. The only reason for a longer delay I can think of is if they want to deal with some bugs first, I can't think of there being any other advantage to them. As others have said, it could be that they're not allowed to mention when the PS3/PC version is coming out as part of their agreement with Microsoft, for as long as the exclusivity deal lasts.
Disclaimer: I don't work for Bethesda and have no inside information etc. etc.

They'd sell more copies of Diablo by releasing it on 360 too, but they havent done that either (yet), and Diablo isnt exactly a game that pushes the boundaries.
I do agree that in all probability, Dawnguard will make it to PC and PS3 at some point. I'm also saying though that they've left themselves an option not to, if they need it.
For all we know, that might be because Zenimax dont want to push more funding at Beth to do it, and instead favour spending money on getting TES:O out.
Yes, Dawnguard will sell on the PC and PS3, but whether it sells
enough to make back the cost on supporting it, patching it etc is another matter.
Another thing they could conceivably do is just wait until the end of the DLC cycle and put it all out then in one block for PC and/or PS3.
Either way though, I guess we'll hear
something on 27 July.