[QUOTE=DMark]
Not letters, but shortened name:
I know people in San Francisco hate it when you call it “Frisco”, but nobody in Las Vegas minds if you call it Vegas. Most people here say they live in Las Vegas, but if anyone says they are from Vegas, it is not like people are pissed off or anything.
[/QUOTE]
By that token, Johannesburg is routinely shortened to Jo-Burg in speech.
As an aside, I sometimes enjoy adding “polis” (meaning “city”) to cities whose names end in “City.”
Examples:
Oklahomopolis
Kansasopolis
New Yorkopolis
Atlanticopolis
NYC is the only place where GWB is rarely thought of as the current President, what with it being the George Washington Bridge.
[QUOTE=Autolycus]
Boston holds its head high above such petty abbreviations.
[/QUOTE]
So the locals don’t refer to Boston as “The BM”?
I’m pushing for Denver to be known as “D-town” but I just keep getting laughed at.
Milton Keynes, artificially created city in England. Known as MK, e.g. in the name of football team MK Dons. Named after economists Milton Friedman and John Maynard Keynes [sub]not[/sub].
London - LDN
Glasgow/Glasgvegas - GLA (Also the airport code)
I don’t think airport codes count. And I’ve never encountered LDN, except in the title of that Lily Allen song, or in BBC London adverts. Nobody says “I’m off to LDN tomorrow”, do they?
[QUOTE=Alessan]
Jerusalem is occasionally shortened - in writing only - to Y-M in Hebrew, or J’lem in English.
[/QUOTE]
I’ve heard Jerusalem referred to as “J-Town” as a nickname.
Isla Vista, the college student ghetto next to UCSB, is known as IV. Santa Barbara is quite often known as SB, hopefully nobody gets it confused with San Bernadino (San Berdoo).
Here in Maryland, we’ve got PG County for Prince George’s County–but no one seems to call Anne Arundel County AA County. 
Mexico City is often called “the DF.” (Federal district)
I’ve heard the entire Dominican Republic referred to as “the DR.” (Yes, I know the Dominican Republic is a country and not a city.)
[QUOTE=Usram]
I don’t think airport codes count. And I’ve never encountered LDN, except in the title of that Lily Allen song, or in BBC London adverts. Nobody says “I’m off to LDN tomorrow”, do they?
[/QUOTE]
I’ve seen it referred to as LDN in e-mails, text and the like, but I guess it’s not as widespread.
Glasgow gets referred to as GLA via text and type a fair bit, it just happens to also be the code for Glasgow International.
Barcelona, aka Barna, aka Barnacity (chiefly among the comic books, fantasy & sci-fi crowd, as that’s what it was called in a fanzine and table RPG), aka BCN pronounced besaena…
[QUOTE=matt_mcl]
Oh, and since someone mentioned states, the provinces of British Columbia and Prince Edward Island are universally abbreviated, in print and speech, to BC and PEI.
[/QUOTE]
New South Wales is “NSW” in writing, and often spoken as /en ess dub/.
South Africa, South America and South Australia are all “SA”, which can be very confusing in some contexts.
Salt Lake City is called SLC.
Belo Horizonte is commonly referred to as BH (“Bay Haga”)
Buenos Aires is BA
Rio de Janeiro = RIO, but I think that’s cheating.
King of Prussia, Pennsylvania is often known as KOP.
[QUOTE=Giles]
New South Wales is “NSW” in writing, and often spoken as /en ess dub/.
[/QUOTE]
(bolding mine)
I’m NSW born and bred, and I can unreservedly say that I have NEVER heard that.
It’s only three syllables in full - nobody shortens it, except sometimes to “New South”, and that’s a Melbourne thing.
You do occasionally hear it disparagingly referred to (by country residents complaining of funding to the big cities) as “Yeah, right; En Ess Double-U: Newcastle Sydney Wollongong!”
Surprisingly those who’ve mentioned T.O. have neglected GTA which I suspect is so commonplace here that it doesn’t stand out. Greater Toronto Area encompasses the surrounding communities, and GTA is easier to say than Torontopolis.
San Luis Obispo is called SLO and pronounced “Slow” or Slowtown.