There is nothing better to accompany a beer than a mustard slathered bratwurst, but mustard in beer?? Believe or not mustard has been used in beer recipes as a remedy for ague or malaria up until World War II.
"There used to be a lot of ague in them days in the marshlands, Marsh Fever as they used to call it, but I have not heard of a case for many a year, except in men comen home from India or some such part. The cure was hot beer with mustard seed boiled in it, which was counted a fine powerful remedy." Gabrielle Hatfield, 1994 - Country Remedies.
The most interesting part about mustard seed is that some have a naturally occurring wild yeast that will ferment beer. In many old world recipes it is suggested to spread some mustard on toast and let it float in the wort to ferment into beer.
Nowadays craft breweries are taking the opposite approach. Breweries like Stone, Abita, and Sierra Nevada have came out with mustard lines that feature their beer as an ingredient. Just recently Stone brewing had "MustardGate" where it was found out there wasn't actually beer in the mustard. Fear not though, I am sure they will come out with the real deal soon. (Story)
If you look hard enough though you will still be able to find the original. One that I hope to be trying soon is Wostyntje, by Brouwerij De Regenboog. There are pretty good reviews for it at Beer Advocate, and it seems that the mustard is not too over powering. It's good to know if I ever come down with ague there is still a remedy available to me.