CS 573 has already reached its registration capacity of 50 students for on-campus students. We cannot increase this at this point because of limited TA support and the room capacity.
But if you still want or need to take CS 573 this semester, DON'T PANIC! You have at least four options.
CS 573 is offered every fall semester. (I do not expect to teach it next year.)
Try again next year.
Come to class, take notes, do the homeworks, go to office hours, and so forth, just like the registered students. During the first two lectures, I will pass around a signup sheet for a waiting list. Students on the waiting list will be registered in decreasing order by homework average. If you do not submit homework (even Homework 0), I will assume you are no longer interested in registering and will remove your name from the waiting list.
Get on the waiting list.
Most likely, everyone who wants to take the class will be able to register by the add deadline (October 1). Just before the deadline, if the class is still full and there are still unregistered students on the waiting list, we will attempt to raise the enrollment cap to let everyone in. The probability of this being necessarily is very low.
We will give priority to students who need CS 573 to satisfy a requirement (graduation, qualifying exam, etc.).
CS 573 is by far the most popular graduate-level theory course, but it is not the only one, nor is it necessarily the most relevant one for your academic/career/research interests. Check out other courses on formal methods, cryptography, machine learning, distributed algorithms, information theory etc. The CS course catalog for Fall 2011 is avaialable here. CS 598 has several sections with many interesting courses.
Take a different theory course.
The distribution requirements for MS/MCS students have been revised recently and there are several alternatives to CS 573 available; more information is available here. PhD students should discuss appropriate theory courses with their Program of Study committees and/or research advisors.
For students admitted before Fall 2010, CS 473 also satisfies the theory distribution requirement, but this option is recommended only for students with serious gaps in their undergraduate background.
You can satisfy any MS/MCS distribution requirement by taking a proficiency exam. You must get a B- or better on the exam to earn proficiency credit. (It's unclear whether PhD students can use a proficiency exam to satisfy a Program of Study requirement; please consult your committee, your advisor, and/or the academic office.)
Take a proficiency exam.
The proficiency exam for CS 573 is just the regular final exam. To take the CS 573 final exam for proficiency credit, just show up for the regular final and write "PROFICIENCY" in large friendly letters on the front of the answer booklet. Your grade will be based on your exam grade, relative to the registered students. For example, if you get a score of 75%, you get the same letter grade as a registered student who got 75% on their final exam.
As a safety net, if you plan to take the final exam for proficiency credit, I strongly recommend also taking the midterms; these will be averaged in with your final. Again, just show up. Dates, times, and topics for each exam will be announced on the course schedule.
Do not take the proficiency exam unless you are sure you know the material! In the past, several students have waited until their very last semester at UIUC to take the CS 573 proficiency exam, and then failed. Those students did not graduate; some of them lost pending job offers; a few even lost visas. Don't do this!
The obvious downside of taking a proficiency exam is that you lose an opportunity to practice with the material. Even if you already know all the material that the course covers, taking the course for credit will help you polish your design, analysis, and communication skills, and possibly expose (and help fill) gaps that you didn't know you had. So again: Do not take the proficiency exam unless you are sure you know the material!