hey all.... I just made the leap and tried to figure it all out along the way.
Gordon's Tech Spot has a lot of great ideas as he has made the migration before you.
http://tech.kateva.org/2008/08/palm-to-iphone-migration-now-address.html
For Tasks: I bought Toodledo for $4 from the Apps Store. It has an import from Palm option. It worked well.
For Calender The long, but free migration goes: export to Date Book Archive (.dba). Import into Yahoo Calender. Export from Yahoo Calender to .csv format. Import into Google Calender. Make an account on Neuvasync and synchronise Google Calender and iPhone.
For Contacts: Was a bit of work, but again, free. Export a single record from Google Address Book to CSV format so that you know how the fields stack up. Then export your Palm Address Book to csv format. Bring both up in Excel, rename the field heading in your address book export so that they are recognized by Google (e.g. Business Phone rather than Phone). Then clean up your entries. Synchronize Google Address Book with iPhone via Neuvasync.
Memos with security: er.... not there yet..... but there are lots of options in the apps store.