I've posted up a schedule of the Rio samba schools technical rehearsals in the sambodromo in 2011.
Carnaval is really late this year, at the start of March, so the rehearsals start late too, in January. They are taking place every saturday and sunday night. Entrance to the sambadromo is free, and the rehearsals are always very popular so get there early to get a good seat. Usually theree is one samba school from Grupo A and two from Grupo Especial per night, followed by a show in front of sector 3. The samba schools parade without their carnival costumes or spectacular floats, but they usually have some treats for the crowd, and its actually easier to see the dancing and choreography without the visual overload of carnival itself.
The list of rehearsals is here: http://www.tdsounds.co.uk/rio-carnival/rio-carnival-2011/rio-samba-school-technical-rehearsals-2011.