I have three issues with this article: It reads like a counter-attack (always disappointing), focuses too much on e-mail as an example, and tries too hard to justify its own existence.
The real risks of having unencrypted webmail in MobileMe are session take-overs (for spoofing or other purposes) and address harvesting. And the biggest overall risk, as far as I’m concerned, is unencrypted contact access - I don’t like feeling that I’m putting 600 people’s privacy at risk just because Apple thought it wise to skimp on security.
(Also, the comments raise a lot of valid points - oh, and Daniel, using encrypted disk images atop iDisk is… pretty much impossible. You try it and see how it goes…)