Getting NFS clients to retry mount if NFS server down when client boots
Solution 1:
NFS is notoriously touchy when it comes to mounts being unavailable at mount time or mounts disappearing randomly. Particularly with older Linux releases, there were some very real and serious issues with both cases. Recent releases have gotten a lot better, but aren't yet perfect.
For this, however, I would strongly recommend looking into AutoFS and the automount daemon. This will cause the NFS shares to be mounted as needed, on demand. The advantages of this include not having to worry about whether the NFS server is available on startup, and the end stations not being affected if the NFS server disappears (provided they aren't actively using it at the time it goes away).
At my previous job, we made very heavy use of NFS, and once we migrated almost all of it over to automounts, it became enormously more useful, stable, and reliable. Also, recent releases of AutoFS (included in RHEL4+) allow you to pull your automount information from external sources, such as LDAP (this is really useful when you have a lot of machines mounting the same NFS shares, as you can make changes in one place and have all machines see the change).
Solution 2:
The cron job solution is one way. Another is to use the automounter (autofs), and mount them on demand, although I've always suffered reliability problems with the automounter.
Solution 3:
Automount should be your choice.