Sorry, but no. I've been on dozens of Basecamp projects and the only way anyone reliably uses it is if they rely on the email notifications.
I really truly believe that some form of project management can and will replace email and it can become integrated into our lives like Facebook my biggest qualm with project management is the project part. People aren't always necessarily working on projects but we sure do work with groups and keeping track of what's going on in all those groups is tough. I haven't seen anything that does it the way I feel it should yet. I'm not talking socially either. I'm talking about a tool that'll let you see what your family is up to or what your church group is doing etc. Yeah, I just described Facebook but the difference will be in how it lends itself to being used. Like if Facebook and Bascamp had a baby.
If you cannot search, read and write messages OFFLINE it has failed in my eyes. Outside of silicon valley or whereever, there are places where you will not get a connection.
Furthermore, it'd have to be more of a protocol rather than a service itself. many companies will not rely on some third party for hosting their most valuable information and communication network. same goes for individuals, probably <1%, but that is the tech-savy crowd...
Along with that we also plan on providing offline support through standalone clients - though this isn't something we are focusing on at the moment. Right now we just want to get the user experience perfected.