Having worked in HR over a number of years so been on the other end of this situation, I think you should call them.
It wouldn't be unreasonable of you, and a decent organisation would expect to appreciate that recruitment is a two-way thing - even in these times of economic uncertainty for the candidates.
They are clearly interested in you from the amount of time and money they have invested themselves in this process.
You could say that you do really need to understand their timescales for the decision because you are continuing to pursue other opportunities (they would expect you to be doing this) - and one of those now seems to be coming to a conclusion - so you would like a decision, please.
This really isn't good practice on their part in that many a good candidate is lost forever by doing this. I can only imagine that, being a large corporation, the decision process is dependent on a number of people inside the organisation.