I contacted Handspring support last night asking if there will be a new version of Treo Mail that will support attachments and (more importantly for me) the new Treo 600. This is the response I received today:

We at Handspring Technical Support do not have the details of the new Treo Mail version. Please sign up for our free eNewsletter at www.handspring.com for latest updates and news concerning our products. Treo Mail works with Treo 600.