Are there any particular issues to watch out for? This is for my development machine so I will be debugging against both SQL Express 2008 and SQL Server 2008. Is the same SSMS used to connect to instances of each or does SQL Express provide its own express version?
There should be no issues. It will be installed as a separate instance which keeps the two installations from interfering with each other. You can connect to the Express instance through the same tools but there will be reduced functionality (no SQL Server Agent, etc...).