Well, since the ledger is public, if you manage your coins yourself, all they need to know are the address(es) that are yours. They can look up what's in them with a copy of the ledger.
If your bitcoins are in an aggregated account like Coinbase (or the now-dead Mt. Gox), they'd probably put in bank-like reporting requirements that make the bank tell the IRS how much money you have if you have enough of it.
Umm, just as with any other expensive property - they ask you to declare them; and if you conveniently forget to declare it, but at some time later (say, after 5 years) they somehow find that out, then you go to jail.
Are you sure that you can hide that stash (and all purchases/deals made from it) permanently?
Thank you. I haven't researched the subject. I still think you will be better of dealing with German tax authorities than with IRS. I suspect there are and will be some European countries that treat bitcoins with more leniency.
If your bitcoins are in an aggregated account like Coinbase (or the now-dead Mt. Gox), they'd probably put in bank-like reporting requirements that make the bank tell the IRS how much money you have if you have enough of it.