I'm brand new to bitcoin, and yesterday I installed the client and fired it up, and then immediately went and bought some bitcoins.  I got an email form the seller, saying "Bitcoins have shipped".  But then I realized that I was behind a firewall (at work), and my client wasn't connected.  By the time I figured out how to move my wallet to my home computer, so that I could connect to the network with the original receiver bitcoin address, a whole day had gone by. Now, I have waited (connected) about an hour, but still don't see the transaction.

Is this normal?  Does the transaction have to be acknowledged by the receiver before it gets added to a block?  If so, does the sender periodically resend the transactions?

You probably don't have all blocks downloaded yet, check your receiving address balance at http://blockexplorer.com/address/insert-your-bitcoin-address-here

Start the client with the -rescan switch. (Or with -? for more commands).

Brilliant. Thanks for the help, the rescan option works.
Thanks for the tip about the wallet backup as well -- already done!