My day job is based around treating and preventing infections. I thought you might like some things clarifying - please tell me to butt out if you want.
There are a few types of Legionella. The one you may have heard about previously is mainly found in water and causes infection. The flu like illness (Pontiac Fever) is it's mild form of infection but in some patients; where a more severe infection occurs and results in pneumonia, this is called Legionnaire's disease, can be severe and in unfortunatley in some cases fatal.
Legionella longbeachae is bacteria that is receiving press attention. We don't have a lot of experience with it in the UK (or worldwide). This is probably not because it hasn't ever caused infections here but because we haven't been specifically looking for it. We know it can cause severe pneumonia and it is likely it has a milder form of illness with flu-like symptoms. Technically, we probably shouldn't be referring to infections caused by it as legionnaires' disease or pontic fever but I expect that we will adopt these terms anyway as the resulting infections are similar and probably indistinguishable without laboratory test. We are finding L.longbeachae in compost but this is unlikely to be the main environmental resevoir of the bacteria - basically we don't know where this bacteria usually grows. We don't know for definite how it is transmitted to humans but in the case of the bacteria on compost it is most likely that ingestion of the bacteria (from compost on our hands) or inhalation of the dust from dry compost is the way we are exposed to it. Simple measures such as wetting down compost before use and washing hands might reduce our risk of being infected by it.
I completely agree with Farmageddun that there needs to be some perspective here - I will still be using peat-free compost if I need to and I see what happens when you get infections on a day to day basis.
Fairygirl ... bird flu is still out there. It's causing biggest problems for humans in China at the moment (new strain, not the one that got all the press attention before the 'swine flu' pandemic but that one is still out there too). There were a few headlines about this but the press have seem to have got bored of reporting on it.