Hi Splat44,
The various Bitcoin clients all use the same definitions for the Bitcoin addresses that send to each other. It is a bit like different email software using the same sorts of email addresses and being compatible.
The MultiBit help might give you a bit more background;
https://multibit.org/help_contents.htmlIn terms of the underlying code it is completely different - it is written in Java instead of C++.
Generally I recommend having separate addresses in different clients. There is the capability to import the blockchain.info backups into MultiBit but this is mainly in case blockchain.info gets DDOSed and people need to access their funds. It's simpler to create new addresses in MultiBit and just send the bitcoin from one client to another.
As I type I think Instawallet is advising people not to send any bitcoin to Instawallet addresses. Whilst their service is unavailable this is good advice I think.
You can always install MultiBit and send yourself a few millis from another client. Try it out and see if you like it. You can run the Satoshi client and MultiBit on the same machine at the same time.