This is very very far away - -- maybe over 1 year before I even start on the project, but I have an idea for something to solve this kind of problem...
Here is the idea:
There is a program you run on some permanately internet connected computer like your home or a server somewhere that downloads, aggregates, and prioritizes communications. Essentially it proxies your communications like Email, IM, web requests, etc. and prepares and compresses them for transmitting to your car in order of importance and available bandwidth to the vehicle. Essentially, it does the 'heavy lifting' of talking SMTP/POP/IMAP/AIM/HTTP and maintaining stateful connections and whatnot to streamline communications to/from the carpc to the absolute minimum needed.
There is another program that runs on the carpc that is always looking for a connection back to the aggregator whether it be through a found wireless network or intermittently via cellular dial-in or over some other type of very low bandwidth long distance link such as amateur radio data like PSK31 (31bps!), or APRS relay. Whenever it can get a connection, it sends up any queued commands or requests and begins downloading the most important stuff.
The goal here is something that can get a usable amount of data in short spurts such as you get when you drive past someone's open wireless network at 45mph.. Given maybe 3-4 seconds in the cell, that gives maybe enough time to get a DHCP request in and a few packets out/in before you go out of range. The timings and protocol will have to be very aggressive and resillient to get this to work but it should be possible.
Anyway, it's still sort of an idea.. it's going to be a lot of work, but I think the project has a lot of potential for resilliant communciations networks (IE not just carpc's). Let's call it "Rapid Response Tokenized Communications" In some ways it feels like it will be like trying to suck a swimming pool through a drinking straw, but still there is some potential.