Can I download the whole block chain from somewhere? Every time I turn on the Bitcoin client it takes days to catch up with the block chain.

Are there alternatives to getting it trickling through the peer-to-peer network? Can I just download it somewhere? Wouldn't this be ideal for distribution via BitTorrent? 

If so, how do I import it into the Bitcoin client?

My client would still be validating this data (can it?), and it is all signed and proof-of-worked, so there is no real security problem here, is there?
Note by Murch: This download link is broken and it seems unlikely to me that anyone still provides this service. Network and synchronization code have been immensely improved over the years so that the initial block download via the network is usually acceptably fast.

You can download the blockchain nightlies handled by tcatm (one of the Bitcoin core developers and the operator of bitcoincharts.com). Unpack and copy&amp;replace blkindex.dat, blk0001.dat files into C:\Users\"username"\AppData\Roaming\Bitcoin for Windows 7, and run bitcoind with the -rescan option. Then it will take only a couple minutes to download up to latest block. Be aware that manually copied blocks won't get verified.