Suresh asked me to expand on a comment I posted here, where I described my experience with the organization of ICALP 2008 in Reykjavik, which I believe is still the most attended and event-packed ICALP conference on record.
First of all, even without satellite events, ICALP is a three-track conference, and has been so since 2005, though typically only two tracks are running in parallel at each point in time. At ICALP 2008, in addition we had 12 satellite events, including the DYNAMO training school for doctoral students. There were no tutorials, apart from the lectures at the PhD school, but we hosted a masterclass by Peter Winkler on mathematical puzzles, which I estimate was attended by well over 250 people, including local high-school teachers and students. Overall, nearly 500 people were registered for the main conference and/or the satellite events.
The workshops were held the day before ICALP or during the week-end following it. They were selected by the ICALP organizers amongst a fairly large number of proposals that we received in response to a call for workshops, based on their perceived scientific quality and on their potential interest to the ICALP community. As ICALP organizers, we made sure that each workshop had a suitable room at the university and some minimal amount of logistical and technical support. (Typically, at least a local student or a postdoc was permanently in residence during each workshop.) We also printed the preliminary proceedings of the workshops and took care of arranging lunches and coffee breaks. The costs were covered by the workshop registration fees. Overall, the overhead generated by the organization of the satellite events was minimal, or at least it looks so two years after the facts :-)
Organizing such a conference was not an easy job, but it was not as daunting as it may seem. In hindsight, I think that it was important to organize the event at the university (not a hotel---in passing, I very much prefer attending events held at universities rather than hotels), to have the assistance of the university support services, of some local students and postdocs, and of experienced conference organizers who took care of the registrations, of the lunches and coffee breaks and of the social programme. Magnus Halldorsson, Anna Ingolfsdottir and I organized ICALP and were assisted by Bjarni Haldorsson and MohammadReza Mousavi in the organization of the workshops. I firmly believe that the task of organizing a conference like ICALP should be shared amongst several people. This certainly worked for us and helped us work more cheerfully, and overcome personal problems, mishaps and periods of crisis and panic that arose during the year before the conference took place.
Overall, I do not think that organizing ICALP ended up being much more work than organizing a single-track conference without satellite events.
Let me close by adding that the model used for ICALP is rather common in conferences related to TCS with a "volume B flavour". See, for instance, the experience of the ETAPS conference series, which involves five major conferences, tutorials and a large number of satellite events, and of the Federated Logic Conference (FLoC), featuring eight major conferences and large number of workshops in 2010. Readers of this post may like to know that typically workshops are proposed by members of the community, and so are the tutorials at ETAPS.
As an external observer, I fail to see why STOC could not follow the example of those federated conferences and, at the same time, broaden its scope to cover more topics in TCS and accept a few more papers, if their quality is excellent, rather than relinquish its high-profile, peer-reviewed status.