[takemikazuchi]

I know of one startup, Compathy(in Japan) that are working the other way around -that is, they're building a product that's first for post-trip sharing and then adding pre-trip planning elements/partnerships to it:

http://www.compathy.net/

I think this is a good strategy since even a small population of traveling power-users who are keen on sharing their experiences can generate a lot of content and value for your site.