If scalability is a non-issue, then why is storing generic data like Namecoin banned/heavily discouraged?
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/1059233
As 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.
Quote
"Premature optimization is the root of all evil (or at least most of it) in programming." -- Donald Knuth