There was some discussion today that started around here: http://www.reddit.com/r/Bitcoin/comments/1yj5b5/unverified_pastebin_gmaxwell_irc_log_mtgox_was/cfkze3p and here's some more explanation: https://iwilcox.me.uk/v/nofrac

TL;DR exchanges can prove that:

• they own specific amount of bitcoins
• and that your bitcoins on their exchange are included in this amount