"[If you manage to score a contract with an OEM to have your software preinstalled on all their machines, I would hazard that you are no longer a non-commercial developer. -Raymond]"
True: you are also unlikely to be a malware developer, though, which is why I thought this aspect was broader than notification icons. (The signing requirement here isn't to ensure that if an OEM starts bundling malware that malware doesn't get a promoted notification area icon!). Also, OEMs are not the only users of unattended installations; corporate, academic and even individual users go this route too. Of course if you're customising the system installation, you could presumably add and use your own root CA anyway.
Signing in itself isn't an unreasonable requirement - I sign my own software, using a Startcom certificate at present (seeing a lawyer on Monday to get a Verisign one).