A colleague (and good buddy) of mine recently posted a blog post addressing the impact of Background Database Maintenance (BDM) in Exchange 2010. This topic is often an area of confusion for people and was worth reposting.
Essentially BDM can be scheduled to run continuously on active databases or scheduled to run at specific times but it runs 24 x 7 on passive databases. Also, the BDM process generates a 5MB/sec Read workload per database so environments with more databases need to ensure that the environment has been properly sized including the network infrastructure between the servers and the storage.
For the full post, check out Paul's Flipping Bits site at: http://flippingbits.typepad.com/ and for further reading, be sure to check out the EMC whitepaper Microsoft Exchange 2010: Storage Best Practices and Design Guidance for EMC Storage for more detailed sizing recommendations when using EMC storage.