UPDATE 21 DECEMBER 2009 Hammerman has posted in the Calculator Inaccuracy for Frenzy Attack Speed topic in the Amazon Basin's Technical Discussion, explaining how Frenzy's attack speed is calculated when the first swing hits and isn't blocked: the break-points derived from the process he describes match those I've found experimentally. Hammerman's post is quoted in its entirety in this post.
There's recently been a debate in the Beginner Frenzy Barb! thread about whether TitanSeal's Attack Speed Calculator was accurate for Frenzy: although the initial argument about whether the calculator and Kyrene's formula produce the same break-points has been resolved, tests reported from this post onwards have indicated that the calculated break-points don't match actual break-points.
Although poison length isn't the most reliable means of measuring attack length, subsequent tests with the mana cost method have confirmed all calculated break-points for Double Swing with various combinations of Phase Blade, Colossus Blade and Colossus Sword (including those with weapon Increased Attack Speed and the known bug when the primary weapon is equipped in the right weapon slot), as well as level 20 Frenzy break-points when equipping two weapons with the same weapon speed modifier and no weapon IAS.
However, when two weapons with different WSMs were equipped or one of the weapons had IAS, the actual Frenzy break-points were different to those calculated (see this post and this one).
It looks as though left weapon slot WSM-right weapon slot WSM is always used to calculate Frenzy attack speed in some way, regardless of whether the primary weapon is equipped first in the left or right weapon slot: it also looks as though weapon IAS on the secondary weapon is taken into account in some way. However, I haven't sat down and tried to work out the actual attack speed formula yet, and more tests may be necessary to gather more data.
I just wanted to bring this to your attention, in the hope that others will test this themselves, either to refute my results and point out any errors, or help to work out how Frenzy's attack speed is actually calculated.