I've been following the development here since March or thereabouts, but if I recall correctly, it's been about a year in works. Snapshots from a quick search:

02.07.2008 - Announcement on the "next big thing" (unspecified): http://www.boonex.com/unity/blog/entry/The_New_Logo_The_Next_Big_Thing

23.11.2008 - What appears to be the first D7 specific message: http://www.boonex.com/unity/blog/entry/Dolphin_7_One_the_way_

26.02.2009 - A status check from late February: http://www.boonex.com/unity/blog/entry/Dolphin_7_development_Update_

12.03.2009 see more - First public preview (build 12):
http://www.boonex.com/unity/blog/entry/Dolphin_7_Hookie_Public_Preview_Build_12_

And the rest of the cycle, from Alpha 12 to Beta 4, most will remember.