Hi,
I travelled Lagos, Nigeria to Dublin, Ireland last summer with a friend who was based in Lagos (I'm based in Dublin) and we used an Excel spreadsheet to keep track of all our information. It wasn't anything as sophisticated as what you guys are considering (automatic download of climate information etc.) It was essentially a large electronic notebook, but was very useful all the same, principally because it allowed my friend and I to share information about the trip and to see what information we had gathered between us, thus preventing us wasting time finding information the other had already sourced.
We tried mailing the file back and forth, but this had problems, because our two copies would get be updated at either end and then have to be merged. Instead I found a freebie website and simply put the file up on their server so it could be accessed by both of us. If one of us needed to add information we would download it, edit it and upload it onto the site again. Sounds like a pain, but it worked well.
It was good to know that all our information about the trip was in one place, and not scattered across sheets of paper in a file somewhere, or under the bed, which the dog was planning to eat! ;-)

We kept the following information (among other things) in it:
Contact details for Irish embassies abroad
Climate information
Running record of actual and estimated spending
Itinerary of all equipment and luggage that we brought.
Spare part information
Vaccination information
To-do list
Shipping info and shippers' contact details
Route waypoints.

On the road, I could always access the website where the Excel sheet was stored, if I needed to. Instead I copied the relevent information (like embassy phone numbers) onto a handheld device, so that I'd have the really important information with me at all times and didn't have to rely on getting to an Internet café.
If you think our spreadsheet would be useful to you or you'd like some input into
what your application should offer, drop me a mail.
Talk to you later,
Ken.