Which bitcoin client distribution to use on a secure pen drive? I have just purchased an Ironkey. Which version of bitcoin client can run without needing installation or download of the blockchain? One which may connect to a server to access blockchain info?

So essentially I can plug it in anywhere and send money direct from the secure drive.
You can run MultiBit on a pen drive in a portable manner if you include Java runtime environments (JREs) for the operating systems you are interested in.  By including a JRE on the pen drive you can be sure it will run on the target computer.  MultiBit runs on Windows, Mac and Linux.

There is some help on how to set up MultiBit on a pen drive here : http://multibit.org/help_runFromUSBDrive.html

You can create a shell script / bat file with the single line: 'local-path-to-jre/java -jar multibit-exe.jar' to start it up using your included JRE.  (Note that Macs are guaranteed to have Java installed so you do not need to supply a JRE for that platform.)

There is a detailed tutorial on how to install a JRE here: http://www.dreamincode.net/forums/topic/42544-putting-java-on-your-flash-drive/

You can store your MultiBit wallets on your pen drive.   By default the path to the wallets is stored in the MultiBit config file multibit.properties as an absolute path (i.e. with a drive letter/ name).   I think you can tweak this to put in a relative path which would then open automatically on any computer.   Alternatively you can just go to the 'Your Wallets' screen and press the 'Open Wallet' button to open it manually.

Strictly speaking MultiBit does keep a local copy of the blockchain but it is the bitcoinj block chain so is less than 20 megabytes.