Hi Sevenblackkittens!
One can purchase hard cases for cellos which are very robust and would probably be ok in luggage hold with a 'fragile' sticker on it .... it all depends on how valuable the cello is - I couldn't imagine Yo Yo Ma's cello going in the hold (his ex-Du Pre's Strad) of an aircraft!
But then in first class they probably have a place just to put valuable awkward items! (especially for celebrities).
I too have heard that an extra seat is often paid for in these cases - makes sense.
If its only a 'cheap' practice cello, the hard case would be just as valuable as the cello! However they cheap ones normally have soft cases, and I'd hate to be the cello if this was checked into luggage..
(have you seen the treatment luggage can receive?!)
Cabin staff are often very helpful and might even sort it all out for you free of charge - I once travelled with a precious Tenor Saxophone (not as big as a cello but not small either) and they found a cubby-hole for it.
I wouldn't dream of putting my cello into checked luggage - tho' something tells me I may never need to!