Read only blockchain in bitcoind , patch ideas
I already did working prototype for one bitcoind (with one copy of blockchain) managing multiple wallets: https://bitcointalk.org/index.php?topic=71542.msg96212 It is IMHO easiest thing to to with current codebase and it would fulfill the goal to provide for multiple users' wallets independently while using resources for only one running bitcoind.

But the discussion around it died somehow, looks like nobody tried the prototype and I got only some private inquiries.