Should be fine and TSA (and security in most countries) shouldn't have any problems with anyone taking prescriptions with them - liquid limits, sharp objects and other regulations be damned! That being said, the amount of prescription medicine you take should be reasonable (i.e. not carrying a years supply or something way more than a diabetic would ordinary carry for their trip).

If you are given any hassle for it I'd ask for a doctor to come on the plane and explain to them how insulin isn't considered an "optional" item for a diabetic.

Safe Travels,

James