Probably disgusting liquid barely resembling beer. I dumped a bunch of old ingredients together once and brew with bread yeast, just because I had some stuff getting ready to go bad. It did not turn out even close to drinkable. But if you catch that sourdough yeast with wort it might turn out differently, instead of using commercially available sourdough yeast. It is, after all, a lactobacillus culture. But I'm no scientist.
Sourdough is a mix of wild yeasts and various lactobacilli. The exact type of each varies region by region. Even if you buy a starter, after several generations it will be overtaken by the local types. That's partly why some sourdough is a good deal more sour than others.
I would agree, though, that it's not especially likely to make good beer, unless you live in Belgium.
after brewing for 25 yrs and keeping a wild yeast starter for bread for the last 5 yrs, what do you think would happen. yes, i did a gallon test run due to the fact that i've done some traditional sour mashes and was just curious. i used about a tablespoon of my bread starter and ended up with a somewhat drinkable brew. it was just an experiment but it came out as something that i would drink, not every day. i got about 60% attenuation and it soured the beer very well. i would have to do a lot more trials to dial in an appropriate grist to produce something that i really dug. i just don't have the time right now but people have been doing this exact thing for a very long time with pretty good success (BELGIANS) and others to name a few. if you have the time i'd say go for it and have fun. i'm not done with my experimentation just yet, as i'm still breathing and hopefully will be 'till at least 2012 - so there's still time!