If you have upgraded to the latest 2.1 firmware, then all iTunes links opened from iPhone’s Safari browser will no longer work.
When you attempt to download an application in Safari, it will show the error
Safari cannot open the page because the address is invalid.
There is currently no workaround, except to inform users about Apple’s bug and direct users to go to App Store and search for the application.
Beta users using firmware 2.2 beta said they are aware of the bug and this should be solved (they can’t confirm due to NDA).
On any case, this is a ridiculous bug from Apple. They tried to patch a security flaw in Safari and end up creating a bigger bug. This is pathetic in software development.. but I sympathize with them (:
Other sources: Cartoon Barry blog