More IPv6 hype
Interesting. So you have 7600 addresses that you aren't using? You are aware that the condition of retaining IP addresses is that the reason for requesting them is still valid? You'll be returning them to the appropriate Regional Internet Registry won't you? As per your contract?
The IPv4 shortage is real. Yes, there are a fair number of unused address blocks (like the 7600 you have), but at the current rate of usage, even if we recovered them all we would only be gaining ourselves a couple of years at the most. Sooner or later a broadband customer will be connected who doesn't have an IPv4 address. V4 - V6 NAT will enable them to reach the V4 world for a while, but isn't a long term solution for the reasons enumerated above. However, content providers are increasingly dual-stacked and all modern operating systems: Linux, Mac and Windows (from Vista onwards) work perfectly with Ipv6 with no configuration required.
IPv4 runout will happen sometime in the next couple of years. Many people won't notice as long as ISPs and content providers manage things properly. But that is the crux...