AirPush was one company that did (does?) sell ads as Android push notifications. On the whole, they're really not that widely used anymore, though - partly because Android users pushed back hard against them about a year ago, IIRC, but also because the newest version or two of Android makes it really easy to identify the offending app and disable the notifications.

If I remember correctly, the pushback was before the new notification tray, so it may have been Android's own response to the problem.

Why do you capitalize "spam"?

My logic was that because they're unobtrustive, they're way less likely to get clicked on, and so sketchy Android devs don't really bother with it.

There was a company for a while that was trying to do that. People exploded with anger (number one, it was hard to identify what app was the cause, number two, it was as annoying as it much be for iOS users). Almost all apps removed it, the company changed strategy if I recall and Google threatened to ban their apps.

But other than one Zynga game, I never, ever get spam notifications and that Zynga game was installed for about an hour or two.