As Mike said, help on "initial headers-only download" would be much appreciated.
Work-in-progress is here:
https://github.com/bitcoin/bitcoin/tree/blockheaders... and my notes on issues that have to be worked out are here:
https://gist.github.com/1059233As for scalability in general: it looks to me like CPU time to validate transactions will be the bottleneck before bandwidth or disk space, so I don't see a strong reason to switching to a 'ledger' or 'balance sheet' method. Effective optimization/scalability is all about identifying and eliminating bottlenecks.