Arbor has released their 2009 Worldwide Infrastructure Security Report and it is an interesting read.  The largest DDoS increased nearly 5-fold from 2004 to 2008 (and doubled from 2006 to 2008) to 49Gbps.  At that size, you definitely need the assistance of your upstream service provider to mitigate.  The report also shows the continuing trend of not reporting/referring attacks to law enforcement.

The report can be found at http://staging.arbornetworks.com/dmdocuments/ISR2009_EN.pdf

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Jim Clausing, jclausing --at-- isc [dot] sans (dot) org