And yes, it has always been dual-licensed, but as Monty himself says: "The basic idea for our dual-licensing was this: if you bought a license then we waived the GPL restriction that you have to redistribute your code as GPL. You could change, modify, extend, distribute, and redistribute the copy in any way you wanted (but of course not change the license of the MySQL code). The license was for any version and usage of MySQL, for now and forever." [1]
[1] http://monty-says.blogspot.co.uk/2009/08/thoughts-about-dual...
Since tax records in Finland are public, you can see how much Monty made from this acquisition (approximately) here:
http://www.iltasanomat.fi/kotimaa/art-1288338449921.html
(Ulf Michael Widenius income for 2008, ~16.8M EUR)