I'm hoping I can get some help on this one too. My problem is similar though I suspect not the same.
First of all, I do know Fiats and Cinqs fairly well so I'll have to say that this problem is not the usual 'they're all like that , sir'.
Basically it is quite difficult to change from 1st to 2nd gear, sometimes others can be a bit awkward but 1st to 2nd is very stiff, it's like you're trying to pull it out of 1st without using the clutch at all! It has also taken to actually crunching out of 1st now, though strangely it doesn't crunch into 2nd [?].
It had a new clutch about 5K ago (due to lack of available adjustment so generally worn out), I've had the gearbox oil changed to try to cure this issue when it first appeared (no improvement)and I've re-adjusted the clutch (there was nothing wrong with the clearance etc anyway but I've given it some more).
My only other observations are that the clutch action is very sharp, almost like 'on/off', the problem is worst when the car is cold and if you try to change gear with engine running at a standstill it goes into and out of all gears without baulking though it feels a bit stiff as though bushes or something are past it. With engine off the change through the gears feels identical so I was not suspecting a dragging clutch (being that it is fairly new and adjusted well)
s**t, sorry this is so long! Finally, even the dealer is a bit stumped, talking of clutch cables, selector forks, synchros and more. I know clutch cable may be the best start but I was hoping to take some justified action, not give them an open cheque-book....[:0]