The official announcement on the japanese homepage, as can be found here:
clearly mentions "2005 年3 月中旬" which translates to "2005 middle of march". That statement was made on the 7th of January undoubtedly taking into consideration the then known progress of the DVD-R DL specification finalisation process.
Since the specifications where only finalised maybe two weeks ago by the DVD-R consortium it seems a bit optimistic for anyone to expect working -R DL media and firmware by now. The question is: Was the finalisation still on time when compared to the estimate made in January?
So if Plextor want's to provide working -R DL support with 1.05 they will have to take the time it needs and that includes any running late of the finalisation process.
Nothing they can do about it.