David Cameron is clear that he won't attend any television debates between the party leaders if the Green Party is not invited; Ed Miliband is clear that such debates should go ahead with or without the Prime Minister (and presumably the Greens). Both, of course, are thoroughly cynical positions. Cameron has no great interest in ensuring proper representation of all political views and parties; he is using the Greens' exclusion as a possible bargaining chip for avoiding the debates altogether. He didn't perform notably well last time (despite apparently thinking he would ace them), knows that they benefit opposition leaders more than prime ministers, and is in particular keen, I would imagine, to deny Nigel Farage any sort of television platform at all.
Ed Miliband, meanwhile, is keen to see the debates go ahead because he knows that it has a chance of offering him a better profile than if they don't; his stance on the 'empty chair', meanwhile, is a natural response to try and embarass Cameron.
There is, though, a good question to be asked about whether we should actually have televised leaders' debates at all? They are not a good fit for a parliamentary system of government; they exacerbate the personality factor in politics (and it is worth remembering that the same media which might decry personality politics is also the institution which encourages it); they were remarkably unilluminating last time and they tend to draw the oxygen of publicity away from more substantive policy issues during the campaign.
The UK's is not a presidential system. The party leaders are directly responsible only to the voters in their respective constituencies. Their leadership responsibilities revolve around appointing cabinets or shadow cabinets and co-ordinating policy across their parties. The party leader who heads his party in government holds a title that denotes his position as 'primus inter pares', not just 'primus'.
Television has its own clear agenda, unrelated to the constitutional niceties of parliamentary elections. Leaders' debates offer higher ratings and an easier way of channelling reportage of an election campaign via personalities, an easier and more popular process. Modern political leaders are notorious for allowing media campaigns - especially television ones - to dictate their actions, and it was the commercial determination of Sky News, along with David Cameron's desire to appease its then owner Rupert Murdoch, which played as big a role as any in setting up the debates in 2010.
If television executives are keen for debates - and in themselves these are clearly not bad things - then perhaps they should consider instead holding policy debates, fronted by the respective party spokesmen responsible. Thus, a debate on health issues with Jeremy Hunt, Andy Burnham and their fellow Health spokesmen. Education featuring Nicky Morgan, Tristram Hunt etc. This at least would be a better representation of the parliamentary system we actually have, and strike against the increasing devolving of politics around the personalities of party leaders. Given Miliband's problems with his own profile, I'm a little surprised he hasn't suggested this himself.
NB: Andrew Rawnsley in today's Observer is very clear that the main impediment to any debates is coming from the Conservatives, and he outlines their reasons for wanting to sabotage them here.