The Stuttgart Tourist Board has arranged a special GUADEC Hotels booking web site, which should provide some discounts, without an additional reservation charge. These rooms must be booked by 27th April 2005.
Reservation is also possible by Phone (+49 (0)711-22 28 233) or Fax (+49 (0)711-22 28 251). Remember to mention GUADEC for special deals. The wiki has a summary of these hotels.
You might also explore their regular hotel search for less expensive hotels which may not be as ideal.
This street finder might help you to see how close a hotel is to the town center.
We have reserved 100 places at the Stuttgart Youth Hostel, in 3, 4, and 6 bed rooms. This special deal costs 80 Euros for 4 nights, breakfast included, checking in Friday 27th May, and checking out Tuesday 31st May. You can reserve a place when you register for GUADEC, while places remain, until May 6th (Deadline extended from May 1st).
If you are familiar with Stuttgart, or if you speak German, you might find a good deal via the various Mitwohnzentrale: HomeCompany, and Zwischenmiete.de.
Please do suggest other options on the wiki.