Most likely starter is listed first with backup in parentheses.
Catcher - Michael Barrett (Paul Bako)
First Base - Derrek Lee
Second Base - Mark Grudzielanek (Ramon Martinez, Todd Walker)
Shortstop - Alex Gonzalez
Third Base - Aramis Ramirez (Jose Macias)
Right Field - Sammy Sosa
Center Field - Corey Patterson (Tom Goodwin)
Left Field - Moises Alou (Todd Hollandsworth)
Starting Pitchers - Kerry Wood, Greg Maddux, Matt Clement, Carlos Zambrano, and Sergio Mitre (filling in for the injured Mark Prior)
Primary Relief Pitchers - Kyle Farnsworth, LaTroy Hawkins, Kent Mercker, and Todd Wellemeyer
Primary Closing Pitchers - Joe Borowski
Extra Relievers and Closers - Ryan Dempster, Andy Pratt, Mike Remlinger, and Michael Wuertz
If you want more info, here's the Cubs Homepage.