I'd love to have an iPhone, but for now the best sync option functionality is available with Windows and a Palm PDA. Still way ahead of anything likely in the works for the Mac for some time to come.
Using Bonsai (
www.natara.com) on Windows (or on the Mac with VMWare Fusion or Parallels) for GTD-type task management (including multi-contexts) sync'd to Bonsai on a Palm PDA (such as the TX which is very solid and has many apps) is simple and powerful.
Having said that, there is a good solution (IMO) for the Mac and Omni Focus... Until the iPhone's SDK has been out and "real" sync-based applications are available, I would just either take my Mac Book with me if I needed to, or print out the information I need before heading out on errands.
One nice thing about the Mac Book (and OSX) is that it goes to sleep and wakes from sleep better than Windows laptops.
Otherwise, someone can end up spending _many_ hours trying to get some way to sync between devices, sometimes a many-step process or a very convoluted process.
I'm glad I looked at this thread because I am going to revisit running Bonsai within VMWare Fusion to sync from my Mac Book to Palm TX. I like the Mac platform (and even better than that my Apple stock lol) so even if for now I am Windows-based, I can still do it on the Mac!
Looking forward to future Omni Plan improvements that would enable me to seriously consider it as my main GTD-type app.