Would it be possible to provide a downloadable blockchain that is updated and verified by a mass of people? As of November 2012, the blockchain is nearly 3 GB in size with a near-linear growth of about 500 MB/month (cut from exponential growth by the hard-coded block size limit, I guess).

I want to install Bitcoin on my average notebook:
Downloading and verifying the latest bootstrap.dat from Bitcoincharts took some hours (5–9, I guess), but that was only 2.3 GB! Now, I’ve been waiting for Bitcoin to catch up on the last 2–3 months, in which blocks have been considerably bigger and each block takes about 5 seconds of being processed, with my CPU at ~25% and my hard drive at max workload.

This is, to be put lightly, unsustainable.

We need a system, in which an already verified blockchain can be downloaded and directly copied to the bitcoin folder, maybe in an incremental form.
The source has to be trusted, for this, one needs a verifying system in which everyone can control the correctness of the blockchain and then sign it, I’d propose to use gpg keys for this.

The source can then be distributed via server-backed torrent, for example.

Is this technically possible and how?

Edit: Since I approached this with a wrong mindset, I posted kind of a follow-up question about the security risks of lightweight clients. Should an end-user download the whole blockchain? Or is a “lightweight” client sufficient?
There already are downloadable versions of the blockchain. There's no problem in principle in having them GPG-signed by several trusted people. However:


It somewhat weakens security. With the software itself you can trust the signatures and you can inspect the code. With blockchain data inspecting it is infeasible.
Future versions will use a different DB format which allows for much faster verification, so there will not be much advantage for a system where verification is obviated but downloading the data is still needed.
In the long-term, end users will not use a full client, but rather (at best) an SPV client which does not need to download or verify the entire blockchain.