Here's another common problem I'm seeing when users attempt to setup interop between OCS 2007 and LCS 2005. With OCS 2007 we fully support an Enterprise Edition Front End server running without a hardware load balancer. However, with LCS 2005 we didn't. With LCS 2005 you had to have a hardware load balancer. Now, I said had to have, but there is actually a workaround. With LCS 2005 you could add a second IP address to the network card. Then associate the pool FQDN with one IP address and the server FQDN with the other IP address. This tricked LCS 2005 into thinking that the IP address for the pool was associated with VIP of a load balancer.
So, the problem comes in when you have LCS 2005 and a single OCS 2007 front end and that OCS front end isn't behind a hardware load balancer. OCS 2007 will work just fine. However, LCS 2005 will not like this configuration because it sees the pool and the server from the same IP address. So, to fix this you'll need to add a second IP address to the OCS 2007 server and associate the pool FQDN to one address and the server FQDN to the other. Once you restart the services, both should play nicely with each other.