They may have had an icey past but the future says that MAN is a major part their expansion plans and I'm guessing by 2010 FR is looking at basing aircraft at the airport.
Although Mr O'Leary may not get on very well with the MAG heads, he does know that MAN will be important, especially if EZY want to get a foot in. You can argue all day that LPL has just as much pulling power for these two airlines as MAN, but to be blunt - it doesn't. LPL will keep on growing and these two airlines will be vital, but if one airline (EZY) started expanding into MAN and not the other, then the latter would be left behind and I think we are seeing this by today's move by FR.
Also, as O'Leary said earlier this year, he has planes arriving sometimes trice weekly and they need to fly somewhere. I also remember him saying that it was vital for FR to enter the bigger markets to keep growing at it's pace, it also mentioned AMS as a possible destination (not from MAN, just in general) but I think they might pass on that one seeing the tax increases by the 'green' government.
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On a completely different topic, not related to todays announcements, I have been looking through the EK website (booked for next summer) and noticed that ythey call themselves the 'World's fastest growing airline.' I know they have aircraft arriving all the time and within the 250-500 seat bracket but are they really growing as quick as FR?