Hi!
I've just booked HI Hostel Florida http://hostelsuites.com/hostel.php?Buenos+Aires+Hostel+Suites+Florida&id_hostel=30&idioma=2. The prices are very good, around 20 US$ per night on shared rooms, 41 US$ per night on single rooms and US$ 46 per night on double rooms (a bed for a couple, I already checked by phone). Likely most people know HI Hostels, but I think it's worth suggesting. The first time I went abroad I didn't know about them.
Buenos Aires also has other hostels http://hostelsuites.com/hostels.php?idioma=2. Maybe locals could inform which are the closest to the Wikimania event place. For me Hostel Florida seems OK.
http://wikimania2009.wikimedia.org/wiki/Talk:Accommodation
Greetings,
Tom