OK, Blockscan (not updated) show on Poloniex's address 15vA2MJ4ESG3Rt1PVQ79D1LFMBBNtcSz1f amount of 13,154 XCP.
Counterpartyd (version 6) show 48,154 XCP, so
Poloniex did get the 35K back after version update.
Can confirm that 6.0 wipes out the relevant transactions:
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2014-02-19-T19:09:09Central Standard Time Block: 286700
2014-02-19-T19:09:11Central Standard Time Block: 286701
2014-02-19-T19:09:11Central Standard Time Block: 286702
2014-02-19-T19:09:16Central Standard Time Block: 286703
2014-02-19-T19:09:17Central Standard Time Block: 286704
2014-02-19-T19:09:18Central Standard Time Block: 286705
2014-02-19-T19:09:21Central Standard Time Block: 286706
2014-02-19-T19:09:22Central Standard Time Block: 286707
2014-02-19-T19:09:22Central Standard Time Block: 286708
2014-02-19-T19:09:24Central Standard Time Block: 286709
2014-02-19-T19:09:24Central Standard Time Block: 286710
2014-02-19-T19:09:25Central Standard Time Block: 286711
2014-02-19-T19:09:29Central Standard Time Send: 0.0 XCP from 19rVQ91AgrYmbpX6Sjxw6qCoP2Q1YFcn5b to 15vA2MJ4ESG3Rt1PVQ79D1LFMBBNtcSz1f (550188a54801105d506fab507995eb8aef15dfcd4d2d2cbf5e6948b831e92b4c) [invalid: zero quantity]
2014-02-19-T19:09:29Central Standard Time Block: 286712
2014-02-19-T19:09:31Central Standard Time Block: 286713
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Some sort of resolution to make the most parties happy still has to occur on poloniex before trading can be opened up. The most agreeable resolution would be for the hacker to return the BTCs, all trades after block 286712 to be cancelled, and the BTC refunded as appropriate. I don't know about the XCPs withdrawn from purchasing at the dump.
Again I should emphasize that such incidents are hardly unique to any coin. Bitcoin had similarily serious issues at a far more mature stage, and Nxt just has a critical issue last week despite a much larger market cap.