There's no way to sync notes or tasks between Entourage and the iPhone, but there IS a solution now for syncing notes and tasks between the Mac desktop and the iPhone. What you need is a) two iPhone apps from a company called Appigo: Todo and Notebook, and b) an account at Toodledo.com, an online task management website.
The Appigo apps store tasks and notes locally on the iPhone and sync to the respective task and notebook sections of Toodledo.com for access from any desktop machine. Toodledo.com's notebook section can import CSV files, so if you can get your old notes from Palm desktop or Entourage into that format, you can migrate them to the iPhone--but you would have to abandon Entourage for notes after that and use only Toodledo.com instead. (Entourage notes are a kind of a pain to export, and require a Paul Berkowitz Applescript and TextWrangler, but it can be done.)
Toodledo.com can also import tasks from other programs, and can even sync tasks to iCal on the desktop. (Conceivably, you could then still use Entourage for tasks, but the sync between Entourage and iCal is very buggy. I have abandoned Entourage for notes, calendaring and tasks.) Toodle.com is free for a basic account, with a yearly-fee option if you want your info encrypted. Also, Toodledo.com now sells its own iPhone app (for tasks only, not notes) so you have a choice of which task app to use on the phone.
I would highly recommend checking out the Appigo/Toodledo.com combo. You will be amazed at how much nicer it is to manage notes and tasks on the iPhone with the Appigo apps than it was to manage notes and tasks on a Palm.
