The ReadyNAS uses the open source project Netatalk to provide AFP. See
http://netatalk.sourceforge.net/Currently in 4.1.7, Netatalk 2.1.3 is used. Hopefully the Netatalk developers will test the developer preview of OS X Lion and update the Netatalk project to use a version of afp compatible with Mac OS X Lion.
Then it'd be up to NetGear to decide whether to do the necessary backporting to support it on the Sparc platform and include it in a future firmware update.
Please bear in mind that NetGear only supports stable OSes, not beta/developer preview versions so tech support won't be able to help you with problems you encounter using it.
If AFP can't be updated to a version supported by Mac OS X Lion then I guess you'd need to use CIFS or NFS instead.