Starring Mike Myers, this colorful co-production between Dreamworks SKG, Imagine Entertainment and Universal Pictures was largely shot in Universal soundstages, including Stage 27.
The Cat also subtly plugs Universal Studios theme parks during the movie (see photo, right) when Conrad mentions an amusement park (58 mins and 45 seconds into the movie).

From the DVD documentary;
The scenes set in the Seussical town of Anville were mostly shot in Old Town, Pomona, California (where some of the paint scheme and stylised signage can still be seen) and the residential area was built on an empty hillside in Moorpark. Simi Valley, California.
Some sections of the town were built in Courthouse Square (see photos below)

Links:

• IMDB entry
• Official Movie Website