Scott used "what's up hot dog?" as a catchphrase from Episode 54: The Wacky Ding Dongs to 84: Impotent Teamsters Unite. On Episode 91: Garry Marshall Presents, "Weird Al" Yankovic asks Scott if he can start using "What's up hot dog?", Scott says this would be fine as long as Al tweets it at least once a day and uses it to open all of his concerts. Al agrees to these terms and has been using it ever since.
In Episode 103: These Times They Are A-Changin', Scott announced his intentions to retire the 'What's Up Hot Dog?' and pay homage to it by renaming Plugs the What's Up Hot Dog Memorial Plugs Section.
Finding a Replacement for "What's Up Hot Dog?"Edit
Since Scott has not been using "What's up hot dog?" as a catchphrase at the beginning of each episode he has been using fan suggested catchphrases, ostensibly to try and find a new regular catchphrase but has had no luck as of yet.