Applying for a PhD / MPhil
How to do a Systems PhD
In case you're wondering what's involved in doing a (systems) PhD, I have a presentation here (microsoft powerpoint) which may be helpful; this is not specific to Cambridge (and, indeed, much of it is not specific to "systems" either ;-).
Interested in doing a PhD here?
The first question to consider is whether you should do a masters degree first. In the lab we have the MPhil in Advanced Computer Science (ACS) which is a general masters degree targeted at preparing students for a PhD. There are also good research-oriented masters degrees at a number of other institutions in the UK and the rest of the world.
My own advice is if you've come directly from a UK-style 3 year undergraduate degree, then doing the ACS first makes a hell of a lot of sense. If you've done a German-style Diplom, or an integrated masters, the ACS can be useful but may not be required. And if you've already done a research-oriented masters, then you can probably jump straight to the PhD. There are no hard and fast rules tho, so worth contacting a potential advisor/supervisor for advice on your specific case.
Applying for the ACS or PhD
Assuming you have some sort of topic, a next step would be to formally apply via the board of graduate studies (web site, application forms). Try to get your application paperwork submitted as soon as possible, even if you need to write "research proposal to follow by email" under that section -- we get a large number of applications every year and getting in early is usually beneficial.
Application deadlines are very early! Although the formal closing date is usually March 31 (to start the following October), most of the funding opportunities (Gates, Cambridge Trusts, etc) have much much earlier deadlines in November, December or January prior to the October you wish to start in. Check out the board of graduate studies web site or contact Lise Gough (email@example.com) for more information.
For both the ACS (option B) and the PhD, one extremely important part of your application is your research proposal. This is an original piece of work by you - typically a 2-3 page document which demonstrates an ability to identify an interesting area, briefly discuss the related work, and to suggest a new approach. This is read by members of the faculty, and is one strong component of how you will be ranked for funding and considered for admission. In some cases an interview, potentially via phone, will follow, at which you will be expected to be able to discuss your proposal in detail. So certainly worth putting some effort into it!