Many prestigious conferences in systems and networking -- such as SOSP, SenSys, and SIGCOMM -- constrain themselves to a single track over two and a half days. This limits the number of papers that can be presented at the conference to around 25-30 at most, assuming 30-minute talk slots.
The problem is that the field has been growing, but the publication venues have not. This means it is becoming increasingly competitive to get a paper accepted to one of these venues. You can check out the stats for yourself here. Although the stats are not yet on that page, SIGCOMM 2009 accepted only 10% of submitted papers. Conference publications in top venues are now a highly prized commodity, and one that is becoming increasingly valued over time. Unfortunately this leads to spiraling inflation, in the sense that the harder it is to get a paper accepted, the more it is seen as a superhuman achievement ("Wow! You got a paper into SIGCOMM!"), causing more people to submit more papers to the conference: a vicious cycle.
This value inflation is evident in the CVs of the hotshot graduate students on the job market last year. Several of us on the hiring committee were amazed at how many freshly-minted Ph.D.s were coming out with multiple papers in places like SOSP, OSDI, and NSDI. Clearly there is a lot more weight placed on getting those papers accepted than there used to be. When I was a grad student, publicatons were important, but nobody pushed me to publish relentlessly -- I submitted papers when I had something to submit. (I'll admit this was not very strategic.) Somewhere along the way the ante has been upped considerably.
Of course, this is a "great" thing for those lucky few who are successful at publishing at these venues. (To be fair, I count myself as part of that cohort.) But it does little to foster the community as a whole. Grad students need publications in top conferences to be taken seriously for faculty jobs. Junior faculty need them for tenure. If the conference venues become more and more selective, I don't see how we can sustain growth in the field as a whole. It seems to me that the number of papers accepted at conferences needs to keep pace with the number of new faculty and students entering the field. Either that or we need to reset our expectations of what constitutes making a research contribution.
There is also a potential loss to the diversity of the research community. Getting a paper into SOSP or NSDI takes a substantial investment of money and time. Research groups with more firepower (like, say, Microsoft Research) have a much easier time than those who might have good ideas but fewer resources. I don't have hard data to back this up, but it feels that it is increasingly rare to see papers from anything other than "top ten" universities and labs in the top venues. One thing that would help would be a quota on the number of papers that a given institution or author could submit to a conference, much as the NSF does for some of its programs. (It seems that everyone I know at MSR is working on at least least three papers for every major deadline. This is insane.)
Now, I am not suggesting that conferences lower their standards. But we need to decide what is reasonable and scale up as the community grows. One way to scale up is to create new venues, but this is not very cost-effective: it is expensive and time-consuming to run a conference, and who has time to go to so many events each year? Accepting a few more papers once you already have the whole program committee in the room incurs much less overhead.
This may mean going to multiple tracks, having shorter talks (my preferred solution), or not presenting every paper at the conference orally. As much as people bemoan multi-track conferences, every "single track" conference is really double-track: there is the track of people who stay in the room and listen to the talks, and the track of those who stand in the hallway and chat.
Finally, it's clear that paper quality varies over time, and it seems unlikely that any specific target for acceptance rates (say, 25%) makes sense for a given conference in a given year. But I think we should take a hard look at where we are headed, and ask whether we shouldn't open up the field more by devaluing the currency of the conference paper.