With Service Pack 1, Microsoft introduced the STSAdm command "Mergecontentdbs". With this command you can move site collections between databases. Unfortunately I recently ran into a bug in this command:
When your site collection contains multi valued columns, it is possible that the data in these columns will be gone after migration.
This issue is confirmed by Microsoft and according to them fixed in the April Cumulative Update. To prevent this issue from occuring, install Service Pack 2 and the April Cumulative Update on your environment.