People I meet regularly ask me if I prefer Jonathan or Jon. I always tell them I have no preference. Most people, and most/all of my family, call me Jon; it's what I tend to go by in a casual environment.
Professionally/officially, I use Jonathan. All business cards, email addresses and resumes display Jonathan. When I write emails to consultants or clients, I tend to sign off, 'Jonathan'. However, once I develop a bit of a relationship with someone, often resulting in more casual messages, I may revert to 'Jon'.
This is not just laziness (though typing three letters is easier than typing eight). It seems to me that switching to a short form adds a personal touch. It signals that you've moved past experiencing someone just as a potential candidate or client, and that you have made some sort of connection with them. Though I spend about 173% of my work day in front of a computer screen, personal connections are important.
More than that, there is an element of branding involved. Professionally, I am not Jon McLeod*; I am Jonathan McLeod. It may seem unimportant, but it lends clarity and consistency to my professional life - two attributes that, generally, I have found quite important throughout my work history.
But getting back to the matter of the personal connection: upon creating this professional "brand", breaking it down and introducing a nickname can be very important. Customers want to feel that they are valued. Dedicated customers want to feel that they are especially valued. There's not a whole lot that I can offer, but a I can offer a personal connection.
So, if you see me, feel free to call me 'Jon'.
*I hardly ever write or say 'Jon McLeod'. The truncated first name preceding the last name always rings quite odd (except when a few people - who have been employing it for years - say it). Just another reason that I pretty much exclusively go with Jonathan in any official setting.