His grounds for doing so are that it's absurd that Labour would be propped up by the Lib Dems if - as the polls seem to suggest - they came 3rd in the national vote share.
That's a reasonable argument. It would be ridiculous for the 3rd place party to provide the Prime Minister - even if Labour came first in number of seats.
However, Clegg is - sensibly - insisting on proportional representation as a precondition of any deal. At first sight this seems to rule out a Lib Dem-Tory coalition as Dave Cameron has said he doesn't want PR introduced. What happens if the electoral arithmetic means that a majority Lib Dem/Tory or Lib Dem/Labour coalition is possible, and Cameron says no to PR, but Brown says yes? It would be surprising if the Lib Dems and Labour weren't able to come to some kind of agreement - even if it meant Brown stepping down as PM in favour of Clegg.
Many would say there's no way Brown would agree to do that but if the alternative is a Tory government (albeit a minority Tory government) then maybe...
Labour have been making mixed messages about PR and the Lib Dems for some time now. In the progressive corner we have Alan Johnson, who sounds very ready to countenance PR (he must be regretting he didn't carry through on the leadership bid last summer - would have made things a hell of a lot easier now, for sure). In the reactionary corner, we have the likes of Ed Balls, who claims that coalition governments are not "the British way". Well that's because of the insane electoral system Ed... but we can actually, y'know, change the electoral system?
A wolf in sheeps clothing is well known tanning salon devotee Peter Hain, who is in favour of Alternative Vote (a complete fudge which combines non-proportionality with being relatively hard to understand, thus being worse than either PR or FPTP) but who says (in the aforementioned Guardian article) that PR is bad because it breaks the link between an MP and his or her constituents "and so makes it more difficult to sack corrupt MPs". Er... been hiding under a rock for the last year, Peter? FPTP really did a good job of rooting out expenses corruption, didn't it? And the "constituency link" is the most overrated piece of bullshit argument I have ever seen. For most of my life I have lived in seats in Essex represented by the most dreadful right-wing Tories - people I have nothing in common with ideologically whatsoever. At least in multi-member constituencies like in the European elections there is a chance of having a Labour or Lib Dem MP who represents (to some extent) your views even if you live in the Tory heartland.
But basically, talk is cheap at this stage and the three party leaders can say whatever the hell they want about PR, coalitions etc... my guess is that if we do get that balanced parliament (I prefer 'balanced' to 'hung' as it sounds like no-one's died, which is in fact the case) then people will be modifying their positions pretty damn quickly. My guess is either (a) Lib Dem/Lab coalition or (b) a short-term minority Tory government followed by new elections later in the year. But we certainly can't rule out the Lib Dem/Tory deal at this stage.