Well, that was quick. No sooner than word got out about Toyota's advertising on the unofficial ModMyi jailbreak store for iOS, Apple has reportedly stepped in and asked Toyota to back down. Apparently the advertising company responsible for the theme and buy called up to remove it and said that Apple had contacted Toyota directly, resulting in Toyota deciding against the promotion to "maintain their good relationship with Apple."
Kyle Matthews, the founder of ModMyi, notes that the deal has been in progress for a while and actually went up about a month ago, so it appears that only now, as the deal has gotten some press, did Apple show any concern about it. This adds an interesting wrinkle to the already strange relationship between Apple and its unofficial software stores. Apple has generally been forgiving of most jailbreakers, though in the past, it has come up with some harsher rhetoric. It appears, though this may come as no surprise, that money is the issue. As soon as a larger corporation like Toyota becomes ready to support jailbreakers, that's when Apple starts throwing its weight around.
It's hard to think anyone else as big as Toyota will touch an ad buy like this now -- there's always a chance that another company will go for the sale in spite of Apple's concerns, but the jailbreak audience doesn't seem big enough to really attract another significant client. We'll have to see what happens -- Apple may have killed this trend just as quickly as it started.