I'm not aware of anybody intentionally doing this, but it's a big old world out there. However, the model organism that I worked with - wax moth larvae, Galleria mellonella - were touted as an answer not long ago as they were observed eating plastic, and I have mentioned it elsewhere on the boards. There is evidence that they are actually digesting it, not just chewing it into smaller pieces, and that digestion is likely to be the result of their intestinal microflora working on it. G. mellonella in the wild eat beeswax, which has chemical similarities to certain types of plastic, hence this digestive ability. I don't know whether the boys at Cambridge are trying to isolate the microorganism responsible; if they are, they're understandably keeping it close to their chests. Lots of Universities investing in commercial spin-out companies these days!
Every living thing has the possibility to mutate when it reproduces - the mechanisms that read the code are prone to getting it wrong. Some errors have no effect, some will have a lethal effect, some will confer advantage and be propagated. However, these are random mutations, and thus the possibility of a particular effect arising is low. If you want to produce a particular mutation, these days we have the technology to do so by introducing the appropriate code, but it will take time and effort as Nature doesn't always want to be told what to do! So yes, it's easier for the virus to mutate naturally, but if you're looking for a particular outcome, it's easier to do it yourself. (For instance, back to bacteria: Yersinia pestis, causative agent of the plague, is very closely related to Yersinia pseudotuberculosis, which is a less virulent pathogen giving rise to gastrointestinal problems. Y. pestis has evolved quite quickly through the acquisition of three plasmids which have given it the ability to survive inside flea guts and produce the nasty bits that make Yp so lethal. However, this 'quick' evolution has occurred within the last 5,000 years - small beer in planetary terms but quite a long time in human perception.)
As to the foot and mouth incident at Pirbright, it should definitely be a one-off - facilities like that are supposed to be very closely monitored so that they don't release things accidentally, and the staff are trained to minimise such a risk. When they were building the BSL3 facilities in our lab, which allow us to hold some nasty bacteria, there was a very heated staff meeting during which a couple of people were extremely persistent in asking how sure they were that nothing would be escaping. I recall my eventual boss having his head in his hands at one point as he patiently explained the measures for the umpteenth time...