Yes, this thread turns up once in a while, both here and alt.food.wine.
To try to summarize past discussions, it appears that the device may have some validity concerning softening tannic wine. Apparently, there are tannins of all sizes, but we only taste ones that fall in a certain range. The magnetic field could possibly facilitate tannin polymerization into longer chains that are not detectable by us.
This appears to be one reaction (of many) that occurs during bottle aging. Does the device really age the wine, no because the other reactions that change the wine aren't occurring, but the wine may appear softer.
I must stress that this is all just theory, no real data to support it.
Andy