bitcoind already uses asynchronous IO. That's not the problem.

The issue came up in a conversation about scalability. If Bitcoins popularity continues to grow, users are very likely to migrate away from running full verifying nodes to lightweight clients, either a different mode of the Satoshi client or different implementations like the Android Wallet or MultiBit.

Lightweight clients cannot verify thus should not relay. And they'll be run by users who just want to send/receive coins from time to time, so don't leave the programs running 24/7. The result could be running out of sockets (like we have had problems with recently). It's especially true because lightweight clients cannot check transactions for themselves. If they want to show transactions appearing immediately (and they do), they have to use "heard from lots of nodes" as a proxy for validity. So lightweight clients are likely to be socket intensive.

We could solve this by just hoping that lots of people run full nodes. The problem is that a full node is quite an intensive thing already, it uses lots of CPU and disk seeks, and will just get more expensive in future. And as transaction traffic increases, that leaves less CPU time available to service thousands of connected clients. The ROI of bringing up a new node decreases at the same time as the userbase increases.

One traditional approach to solving this is frontend proxies. Jabber.com/org used this technique many years ago, and Google has also used it to scale up the lockservice (see section 3.1). It's effective because often maintaining connections to thousands of clients doesn't involve much brainwork, just shifting bytes around. This is especially true of Bitcoin. So if somebody is running a full node already they could increase their client capacity by just bringing up a frontend proxy and having it handle things like outbound tx broadcasts/deduping inbound broadcasts, connection setup, relaying recently found blocks etc. A well written proxy could probably support tens of thousands of simultaneous clients which frees up the bitcoinds time for verification and wallet manipulation.