sigrun linked to this Carter Bays (HIMYM) interview, and it includes a link to an NPH interview, which includes this gem:
TVGuide.com: What say we work musical theater into Barney's story line?
Harris: [Laughs] I'd love to do a big musical episode of How I Met Your Mother. Alyson Hannigan did it on Buffy, and Jason [Segel] can sing.... Cobie Smulders is taking voice lessons, I know.
TVGuide.com: You need to make that happen. It's going to be a thing, I tell you.
Harris: It's going to be legen — jump the shark — dary.