Rush might not be sure which way to go on air about this issue. The jokes might indicate where his sympathies lie, though.
It's not a radio-friendly issue. It's one thing to see evidence of forgery, but on the radio, he'd have to describe it. The other aspects of the issue are complex and/or lengthy. The grandmother saying he was born in Kenya; the Mombasa birth certificate; the news stories that said Obama was Kenyan-born; the unresolved questons about whether Obama was adopted by Lolo Soetoro and is legally named Barry Soetoro; the stonewalling by the Department of Health in Hawaii despite having legal ways to confirm the legimacy of Obama's alleged COLB; Vattel, Blackstone, Wong Kim Ark, Minor v. Happersett and how to define natural born citizen; Obama father being a British subject/Kenyan citizen and how that naturally descended to Obama .... there's just too much involved to get to the point quickly and succinctly. And it's not really a good TV issue either, for the same reasons.