Keaton played Bruce Wayne and Batman in 1989's Batman, the first modern big screen version of the cape crusader.
The site goes on to speculate that Keaton is town for one of three reasons:
1: Michael Keaton loves Chicago and is having a good 'ol time hanging out - eating pizza, burgers, beef sandwiches, hot wings and watching The Bulls win game one of the Playoffs.
2: Or maybe Keaton is in another movie, which is also filming around the same location?
3: But maybe, just maybe the ex-Batman (and still my favorite Bruce Wayne) has a cameo in the new Batman film - The Dark Knight, which is shooting under another name that we cannot reveal
The Dark Knight will be released on June 18th, 2008.