The most recent confiscation is of 144,000 bitcoins (around $29m at current prices). The previous amount was 26,000 bitcoins (around $5m at current prices).
There's no question that Ulbricht has been detained by the FBI and has some major problems on his hands, he will almost certainly go to jail for a very long time, PopeHat estimates jail time of around 10 to 30 years.
The question though is what does 'confiscation' mean here? The FBI may have encrypted wallets that contain the private keys to control these bitcoins but without the key to access the wallet they are useless. I could stand outside Fort Knox and claim to have 'confiscated' all the gold within it. Its within my sight so I've confiscated it right? Except I can't actually get at the gold right now... and if the US government wants to come in and move that gold somewhere else then I can't stop them.
Both of those conditions potentially apply to Ulbricht's bitcoins. Bitcoins aren't store anywhere, they are part of the network and blockchain. The real 'owner' of the bitcoins is whoever knows the private key to make transactions with the wallet that holds them.
If the FBI doesn't know that private key (because its encrypted and they can't crack it) then they don't have any more control over the coins than you or I. If Ulbricht has memorised or has a copy of the wallet seed then even now he retains control over the bitcoins (although his ability to exercise that control is no doubt currently limited).
Whether this changes in the future will likely depend on what mistakes Ulbricht made, what precautions he took and what leverage the FBI has to get the keys off him but it's quite possible, maybe even likely, that the FBI hasn't confiscated anything.
The speculation is that maybe the FBI has the encryption keys but is keeping the bitcoins in the same wallet but given the architecture of bitcoin this would leave them open to removal by anyone else at any time. If Ulbricht did know the wallet seed he could get a message to someone to move them to another wallet and the FBI would lose them - why would they take that risk? Instead it would make more sense that if they had the keys they would immediately move them to an FBI-controlled wallet.
I'm no fan of Ulbricht or Silk Road but in the past it hasn't been possible to claim that you've seized assets of a criminal without actually having control of them. The new-to-the-world architecture of bitcoin and, perhaps more so, the relative lack of understanding of that architecture make this possible.
Whether the FBI is using the term seize in a meaningful way or whether they will find in the coming years that their 'seized' bitcoins start disappearing through their fingers will be interesting.