You may need a visum to enter Switzerland; please check the Swiss regulations. Note that Switzerland is part of the Schengen area since December 2008, that is, a Schengen visum is valid to enter Switzerland.
On request, the organizers send to you a letter of invitation for ICALP 2011, provided that you are author of a paper that has been accepted to ICALP 2011 or - at least - you could have been such an author, which most likely means that you are a more or less established member of the TCS community.
In such a case please first check with the embassy or consulate where you plan to apply for a visum, whether you need a letter of invitation and - if so - in which format. Some embassies require the letter to be sent from us to them directly, whereas in other places you have to provide a (paper) letter that we have to send to you via snail mail.
Once you know what is needed, please contact the organizers, for instance, via icalp11@inf.ethz.ch, and provide the following information:
