Little Springs Design has an essay:
Desktop applications and web sites have since the 1980s been designed assuming multiple windows. If you want to get information from another application, just open it. Browsers can have multiple windows open. This is particularly important in our current world of online applications: I can have my email window open, my business networking site open to research somebody in my email, and my calendar open – all in separate windows. Mobile phones do not support this cross-site fertilization. Instead, only one window can be viewed at a time.
For one web can become a reality, browsers must become adept at handling multiple tasks. This, by itself, is inadequate. High-end phones (variously called smart phones and PDA phones, usually with an operating system like Symbian, Windows, or Palm) have rudimentary multi-tasking – but on an application level. Multiple browser tasks must become easy; switching between pages must become easy; split-window viewing must become possible.
Browsers must become adept at handling multiple simultaneous tasks in the same way that messaging applications are adept at handling multiple conversation threads, except that users will want the information either simultaneously or with rapid switching. The browser will have to be re-engineered, from the ground up, to truly embrace the mobile environment rather than being a miniature desktop browser.