Database Software

Please bear with me. I'm new to Oracle clustering. Need some expert advice.

Our shop has two app servers running OAS in a manual failover config. We also have two database servers running Oracle 10g, also manual failover. Mgmt wants to set up RAC and also cluster the app servers with BigIP handling load balancing. Answers to three questions will help to resolve some design issues we are wrestling with now:
1) Will we have two separate clusters, one at app tier and another at DB tier? Or is it possible, if not practical, to have one cluster for everything? Seemed like we would need two since RAC and OAS appear to have their own different needs. Mgr thinks we can get by with one. I say they don't mix.
2) We are creating a VLAN on one of our high-speed switches for node-to-node communication in RAC. Can or should we share this VLAN with the clustered OAS app servers? My guess is we should keep this VLAN for RAC use only, and add a second one for OAS comms. Our network can be pretty slow at times with heavy traffic. Boss wants to know how we can isolate traffic onto our own VLAN from BigIP to OAS cluster to RAC. Any suggestions?
3) OAS and RAC will both be pointing to our SAN for shared storage. Each will have its own LUNs. Is it possible/adviseable to share a LUN for things like log files and backups? Or should log files and backups be kept on separate LUNs between OAS and RAC? If OAS is in a different cluster than RAC, then wouldn't a single LUN get clobbered?
Grateful for your input. I'm reading my RAC and OAS books every night to ramp up quickly. :>)
Ron