[It's been slow posting as I've been very busy at work and I also recently moved flats and am still lacking internet access.]

This past Monday was my 6-month anniversary as a resident of London.  Following are a few learnings from the expat point of view and in no particular order:

• If you have a Blackberry, download the Google Maps application.  That will save you from having to bring around the London A to Z book of maps.
• You will have no credit history so be prepared to have to go through hoops for things like credit cards or phones.  I've had a harder time getting a monthly contract for a phone than setting up a bank account or credit card.  If you bank with Citibank or HSBC in the US, they will have departments that specialize in expatriate banking. Phone these departments up and ask them to set you up with a UK bank account.  Getting one of these is key to other things like utilities, leases, etc.
• Customer service isn't what it's like in the US.  Get used to it.  It will be hard and you'll have times when you'll want to tear your hair out.
• They drink a lot more here than in the US. 
• Love the lingo:  Duffer, Cock-up, Rom com, yummy mummy, just make sure you know the meaning.  Oh, and the term "double-fisting" is NOT a drinking term for when you have a beer in each hand.  Luckily I did not learn this the hard way.
• Most of all, there are so many things to do and places to explore.  Get involved.  Check out Time Out magazine or sites like View London.  Sign up for Urban Junkies, which delivers you a daily dose of what's on tap around town + a weekend guide every Thursday. 
• Related to the above, get an Oyster Card and, as much as possible, get the unlimited use option for a week or month.  It will force you to go about town to 'get the most' out of the money you've paid.