hit home button while on a skype call on a jailbroken device.
That surely DOES look like a bug and not smething made on purpose by Skype.
Obviously, it's a "bug" that affects only jailbroken devices so as far as they are concerned, it's not a bug at all.
Skype have always tried their best to spoil the jailbreak party. It seem's likely that if Skype detects the phone is jailbroken (it can) it will disable backgrounding.