Mother Jones has a summary of the most terrifying comment on a discussion thread you've ever seen (the entire comment can be read here). It's from someone called Doug R, at The Oil Drum, a forum for petroleum geologists and oil professionals (which must be experiencing a surge in popularity these days). Doug R's is not the only view on the thread, but the site editors think he sounds authoritative and that this is unlikely to be irresponsible scaremongering (UPDATE: though see comments below). He argues that - based on the limited information we have - it's likely that the well pipes below the sea floor are broken and leaking, which means capping the gush is impossible:
"It's a race now...a race to drill the relief wells and take our last chance at killing this monster before the whole weakened, wore out, blown out, leaking and failing system gives up it's last gasp in a horrific crescendo.
"We are not even 2 months into it, barely half way by even optimistic estimates. The damage done by the leaked oil now is virtually immeasurable already and it will not get better, it can only get worse. No matter how much they can collect, there will still be thousands and thousands of gallons leaking out every minute, every hour of every day. We have 2 months left before the relief wells are even near in position and set up to take a kill shot and that is being optimistic as I said.
"Over the next 2 months the mechanical situation also cannot improve, it can only get worse, getting better is an impossibility. While they may make some gains on collecting the leaked oil, the structural situation cannot heal itself. It will continue to erode and flow out more oil and eventually the inevitable collapse which cannot be stopped will happen. It is only a simple matter of who can "get there first"...us or the well."