Infinsipan requires Java 6 and higher so Java 5 wont work.
We're planning to make several Hot Rod clients, and the client does not depend on Java6. The current Java client does, but it could be adapted.
As you might know, there are Ruby and Python clients, and you could also use Memcached libraries: I guess there exist implementations compatible with Java5.
Using the Memcached protocol is not as efficient as the custom protocol as it's not smart in finding out which nodes to connect to, but you could start using the Memcached protocol and then migrate the Java code to Java5 if needed.