If the application is bad it may not be preferred. So it should be covered by the target userbase. I mean, this must be relevant in the context.

Ask telcos about branding

In About box can be shown, in credits.

In the application name.

In your signature here.

Really, everywhere and nowhere, but if application is open source, expect a debranded version. Also, keep in mind you may want to not have people use your branding. See the Firefox/Iceweasel debacle. If it doesn't piss off the user I don't see such happening quickly though. This is a dilemma in advertising: you want to advertise and get user's attention while not pissing user of.

Hm, thats it for now from my side... not too creative/bright right now... :/