I need my tasks available whereever I am, like my mail (and bookmarks and passwords and ...).
OS X Mail has Notes and To Do task entries that can be stored on your mail server but I don't believe any other client recognizes them: on my IMAP server they're stored in a Maildir called ".Apple Mail To Do" and encode information (priority? due date?) in a base64-encoded blob with MIME type application/vnd.apple.mail+todo.
Google Mail has a simple Tasks feature with multiple lists, due dates and such -- probably enough, and available anywhere you've got net. They doc how you can put it on your iGoogle page if you're into that; I am, but I wanted something smaller.
Fluid lets you create site-specific desktop applications from web sites. Fire it up then enter the URL:
I first tried .../iphone but it complained "This version is not compatible with your mobile phone" and suggested the Andoid link.
Google provides more details on accessing your task list from iPhone and Android phones.
I wish I knew about that one when I had my Treo phone. It throws an error about "Entity 'copy' not defined" but seems to function.