Separate wallets sharing the same blockchain? Since the blockchain is such a large file, I'd like to have more than 1 account on the same computer share it.  Is that possible?
The most straightforward way to do this with Bitcoin-Qt/bitcoind (at this moment) is to swap the wallet.dat in your bitcoin data directory when the client is not running.

Another method (if you're on a UNIX-ish OS) is to create multiple data directories, then symbolic link (ln -s) the blocks and chainstate directories (not database) inside your datadirs to a common data dir. 

mkdir ~/.bitcoin-alt  # new wallet directory
cd ~/.bitcoin-alt
ln -s ~/.bitcoin/blocks ~/.bitcoin/chainstate .
# launch new wallet
bitcoin-qt -datadir=$HOME/.bitcoin-alt


Then you can leave the wallet.dat alone. When launching you can specify a -datadir based on the wallet that you want to use, and it will share the blockchain dirs. 

However even with this solution cannot run two bitcoind/bitcoin-qt instances at the same time sharing the block chain! This is currently not possible and will result in corruption if you try.