In my mind the $750M valuation is still actually pretty consevative. GitHub has a long long way to go and I see so many great things ahead for the team, but I don't understand why they would raise $100M?
Is this a deal for the founders to take some money off of the table? I don't see what benefits $100M will have for them.
(Don't shoot me for my opinions on this one), but I think it's also really only a matter of time before the legacy systems move from SVN/Mercurial/CVS onto GitHub. Some people use BitBucket (I guess because Atlassian makes some other great software). SourceForge (past it's sell-by-date), and Google Code isn't really suitable for anything other than hosting specific release candidates or tags..