Is this compulsory when taking a corner?

[FFCpmd]

I don't think I've ever seen any player take a corner without putting his arm in the air. Why? Is it to let the rest of the team know who's tasking the corner? I'd honestly like to know!

[cookieg]

It's a signal for which corner routine they are going to be playing, apparently.

[H4usuallysitting]

They're checking the density of the air in relation to the armpit

[btffc]

It's a signal for which corner routine they are going to be playing, apparently.

Correct. At its most basic level, one arm(say the left for this example) means the ball is going near post and the other(right) means far post. Then the players in the box know where to make their runs based on where the ball is going to be delivered. It is also about timing. The drop of the arm is the signal to start the routine. I'm also sure it can be more complicated than near or far post at the PL level with other signals you don't notice indicating specific routines such as the number of fingers held up.

[gang]

It's absolutely ridiculous, each club seems to do the same thing, do they think the other team aren't watching?
Having said that I did notice Silvas reaction when we scored from a corner and reading his lips was quite easy when he clearly said "It worked."

[cmg]

It's a signal for which corner routine they are going to be playing, apparently.

Absolutely right.
In my day it was both hands in the air: Equals - 'I've no idea. Could be going bloody anywhere.'

Suprise tactics, always the best. If I didn't know, how could they?

[Alec of good Stock]

First time I can remember a Fulham player with arms in the air as they were about to take a corner was John Beck in the late 70s.

[ron]

It's signalling "indirect" meaning: "Don't expect this to go straight in....."

[Jim©]

When I played, we had different signals like stopping during run up and moving the ball a bit with your hand, hands on hips before taking etc. Watch corner takers, it's definitely not just the obvious hand up routine.

[bog]

Is it to signify which side of the pitch the kick will fly over all their heads from? 

[_Putney_]

It's demonstrate that it is in fact a corner.

[Southdowns White]

I thought it was the short players checking the wind direction.

[H4usuallysitting]

It's usually a signal to say.... I don't think I can get the ball past the first defender

[Steeeeeeeeeed]

They are waving to their mummies.

[Eton White]

It's to apologise to the receiving players and fans in advance.