Like many Getting Things Done (GTD) and productivity oriented folks, I've been looking at web-based solutions (Ta-Da List from 37signals is looking most promising so far) but I've become very fond of iGTD and am not prepared to completely overhaul my entire workflow just yet. In poking around today, I found a good short-term workaround. Drag any task in iCal (which iGTD syncs beautifully with) to the calendar and make it an all day event. Sync to the iPhone and your tasks are now accessible in the iPhone Calendar. You lose the categorization iCal/iGTD makes available as the iPhone doesn't do the multiple calendar thing but it's workable in a manual sort of way.
Update: Or, as just reported by TUAW, you can plan on using OmniFocus which will, according to their report, include its own embedded webs server to generate an iPhone-friendly display with no syncing required. I've ben holding off jumping into the beta of OmniFocus but this may tip me over the edge.