That's what mtgox said too, I believe their figure was 98%:

On average 98% of customer bitcoins are held in cold storage...Offline wallets are generated from an offline system and kept in paper format in three separate locations, using a technology based on raid.

https://bitcointalk.org/index.php?topic=23938.40

http://www.managementtoday.co.uk/bulletin/mtdailybulletin/ar...

But of course, they weren't really in cold storage or offline after all, because somehow they all disappeared...

Not that I mistrust coinbase specifically, but without insurance, audits and regulation (i.e. without a banking license), I'm not sure I'd trust anyone to store significant amounts of money.

Banks that hold large amounts of cash either insure it against fire/flood/theft or self-insure with other assets - what is Coinbase's plan if these cold-storage coins are destroyed, either by malicious attempt or disaster? That link shows the coins are locked away, are apparently not accessible, but how safe are they against actual loss?

Gox claimed 95% of their coins were in cold storage.

Unproven claims aren't worth much.