OSX 10.7 "Lion" issued a new version of AFP which is not compatible with devices that use older versions of AFP. Our older NAS devices all include the previous version of AFP. In order to update them to the newer version to comply with Lion, we would need to redistribute an entirely new OS but the devices were not designed at the time to be upgraded in that way. Thus our R&D department decided to focus on current models, which have all been upgraded to meet with Lion specifications. We realize this is a frustration from many of our customers who have not needed to update their NAS.
Fortunately you can still use an older NAS with Lion via SMB or FTP protocols, however Time Machine will not function because its locked into using AFP. There are other backup methods such as Intego or merely dragging and dropping which are still available.