I have had a fair amount of luck using Highrise by 37signals as my platform of choice. Although this is a CRM tool I found that targeting this at my internal customers gave me a seamless way to organize based on both customer and context. By context what I mean is using custom defined Categories to replicate the GTD @context framework to create a very usable GTD-inspired system. This functionality transitioned perfectly to the iPhone and allowed me to have one application to effectively manage my tasks among multiple customers and multiple contexts.
So far this has been absolutely awesome. Truely cross-platform and 'hosted/cloud based' data available anywhere without manual intervention.
My current challenge is that I am doing more Project and Team based work that doesn’t necessarily correlate with a particulate customer. Highrise gives me the ability to sort this work into what is called a ‘Case’.
Potential deal breaker – I cannot create a new Case or - more importantly - assign new tasks to a Case from the iPhone application. I'd really like to do this without reverting to the browser interface. This way I can capture all my takeways - both personal and team/project oriented - using one tool and one interface.
Perhaps I should ask 37signals first. I just created a ticket with this question.