How to make NFS mounts available while offline?
Solution 1:
http://www.linuxpromagazine.com/Issues/2009/99/Offline-FS
Here's some information on various alternatives to do this. Primarily discussing OFS. I've not used it myself, but it seems more mature than my prior suggestions and specifically designed for the task rather than trying to speed up NFS access like CacheFS.
When I need to keep two things in sync, I usually use a Subversion repository as an intermediary between the two systems, but this can be labor intensive.