I'd always say 'YES' - our condition is covered by Disability Discrimination Act & Equalities Act's etc among others after all...
There seems to be an awful lot of negativity towards the word 'disabled', as if ticking that box as a 'yes' somehow makes us less of a person or that there is something 'wrong' with us that is made worse just by ticking a box? I don't believe this to be the case, it simply allows the employer to make an informed decision about the applicant - if there are long terms plan to develop the role in to something that an employer knows would put a diabetic persons health at risk, or the nature of the role/environment would not be conducive to the applicants health then they have a right to know that information up front AND a responsibility to the applicant to consider it before offering interviews etc. There's also a bit of respect & courtesy for other applicants, if I was offered an interview, then upon getting there found out things about the job that I realised as a diabetic meant the job would no longer be suitable for me, I personally would hate to feel like I'd wasted an employers time, as well as taken away an opportunity for someone else who wouldn't have had to turn the gig down for health reasons =S.
Another point to consider is this, whether You, I or WE consider it a disability or not, as it's covered by DDA so someone, somewhere, who writes guidelines for what is a disability and what is not has decided it is. That makes it *almost* a solid fact. One I think the majority of us are aware of (but may not agree with) By consciously ticking 'no' you may not be lying as such, but you are also not telling the 100% truth as far as 'the rules' are concerned. So to take the point a lot of people have made that you 'would tell them later as I have nothing to hide'...if I were an employer, and I interviewed a person who had ticked 'no' on their application, who then let me know they were a diabetic and had been at the time of filling in the form, my first thought wouldn't be 'wow, how open, honest & upfront their being about their health issues'...it would actually be 'what else are they hiding/not telling me in their application if they hid something this important up until now?'...
...that may just be my paranoia kicking in to overdrive but that's just my views on the above, I'm not saying either side of this coin is right or wrong as such, but I do disagree with anyone who'd tick no.