king Ralphred,

Thats because those devices unable to connect don't support connecting to Ad-Hoc WiFi networks out of the box. The nexus S OTOH shares a hotspot via infrastructure mode not Ad-Hoc hence its visible to all devices. For the nexus S to connect to the N900's hotspot there is a tweak on xda that enables Ad-Hoc connectivity. For the Pi I'm not too sure as unfortunately I haven't got my hands on one yet...