The exchange rate today

Very Uneducated Guess Follows, use at your own risk

I'd say it would have to do with the world financial markets... you exchanging up to, maybe at most $1000 for a short time period doesn't have the impact on the global financial as opposed to major money trades of thousands/millions of dollars/pounds.

Could the "tourist rate" really be the buy rate?  In other words, it's the rate that a bureau de change will give someone for exchanging from a currency.  If I go to a bureau de change, I will have to sell my £ for lower than the exchange rate (and get $1.42 instead of $1.49).  However, if I wanted to buy pounds, I'd have to pay the higher ($1.49) rate.

Here is an explanation:

http://money.uk.msn.com/banking/exchangerates.aspx

Interesting read Sweetpeach.
The one thing I don't think they mentioned and is very important is what is the conversion rate on credit card transactions. I think the answer is it is different with each cc company.

They did touch on it very briefly at the end of the article.  MSN mentioned that the rate that most credit cards offer is closer to the market rate but that it varies depending on the credit card.  I find that the credit card rate HSBC offers and their ATM withdrawal rates are generally competitive and usually better than the rates offered at the places in the airports. 

That's my experience also.
I once made out like a bandit when the rate went down (or was it up) anyway by the time I got my bill I paid less than I would have if I paid cash for the item, if you get what I mean.
That was many years ago and I think they got wise to that and changed their methods of calculating. Now they can post to your cc account in a couple of hours.

If anyone is interested, we did a few transactions withdrawing from a Bank of America account at a Barclay's ATM and the rate we got was about £=$1.49.

As of 1401 Central Time, 3-12-2008:

£1 = $1.4805

This link will give you the accurate exchange rate each day from google:
http://www.google.com/search?hl=en&q=convert+1+gbp+to+usd&btnG=Search [nofollow]

Actually, I think if you go to an actual web site, it's more accurate.  xe.com has it at $1.47475 currently.

Any of the finance sites update frequently (like Yahoo)
The regular Google search rate is updated semi-frequently.  I'm sure there's a "live" feed somewhere, but that's way overkill.

yeah, I don't know the specifics on google, but then again, I don't really concern myself with it right now, as a) I'm not going to the UK in the near future and b) there's nothing you can do about it

Any of the finance sites update frequently (like Yahoo)
The regular Google search rate is updated semi-frequently.  I'm sure there's a "live" feed somewhere, but that's way overkill.

I use:
http://www.x-rates.com/d/USD/table.html
Good luck and MC