To get info on what's been done so far, look back in the diary postings here. Also log in to the IRC channel #otherpower and irc.anotherpower.com Don C is usually logged in and happy to explain what he's done and planning to do.
Basically, he's finished building a full sine wave inverter output section and drive/control section. Last I knew, he was working on a DC to DC converter design to act as a front-end to the inverter. The idea there is to allow a wider range of usable input voltage to feed the inverter. As an example, 12, 24 and 48 volt inverters each accept a fairly narrow range of input voltage around their rated value. What if an inverter could accept the entire range of say, 10V to 60V? Or 20 to 100V? I suppose you get the idea.
From a purely electronics perspective, adding a DC-DC converter front-end adds some inefficiency (more losses). But it has the potential, depending on one's energy input resources, to increase usable energy production by far more than its losses. Let's take a 24V, grid-tied wind mill as an example. To keep it simple, we've got no batteries. A stock inverter might accept an input from around 18V to 30V. In low wind conditions your mill might be putting out 8 to 16V... too low for the inverter. In high winds, it might put out 30 to 50V; too high for the inverter. A DC-DC converter designed to handle your most common range of supply V might be able to feed your inverter a steady 24V across all or most of that range.
A DC-DC converter could also be used in battery charging applications to reach "cut-in" or charging voltage sooner... only got 10V? The DC-DC converter could boost it to 14V (at a lower current) and you're charging. Ideally, the circuit would switch itself out when the source voltage is in charging range, to increase the efficiency.
The "boost" circuit that jacquesm is working on (suggested by Victor and apparently done in production by Bergey) takes the DC-DC boost converter concept and uses the windmill's generator coils as a part of the boost circuit. It lowers the cut-in speed by boosting whatever voltage is available to charging level.