The March issue of Linux Journal has a piece on OfflineIMAP. It took a bit of tweaking to get it to run on the older laptop my employer provided but it does work. Makes it very convenient for me as I subscribe to a lot of mailing lists and often don't have the time to sit at home to read through them. Being able to sync the laptop to multiple mail servers and work offline is an awesome ability.
A nice coincidence that fits in nicely with the project is that I do not use the default inbox. I use Procmail as my incoming MTA and anything that passes all the way through those filters (SpamAssassin, SpamBayes, topic sorting, etc) gets filed in a different inbox folder. It took a long while to sync initially, due to the size of my e-mail archive, but updates are quick enough.
My thought for its use: fire up the laptop, start the sync, go build the coffee for the morning, and take both to work a few minutes later.
With a bit of tweaking, I can see this used as a way to maintain mail backups.