Stumbled upon a competitor --
http://rememberthemilk.com/
it has an iPhone interface via Safari at
http://i.rememberthemilk.com/ (for $25/year though)
Didn't find many needed pieces there though -- syncs only given "lists" to iCal, has lists and tags, which you can use to implement contexts, and other mismatches with the GTD as we used to know it. Anyways -- for those who used RT[no F!]M, what's folks' impressions?
Apparently it syncs the todos to iCal via webcal://, which is useful -- that's how I found it, trying to sync my Zimbra todo's to something Mac'y/iPhone'y.