Solar Windows
For what are they good?

Solar Windows are windows that generate electrical power. They look and feel like every other windows and they also behave like a solar electric photovoltaic panel

Solar Windows and Building Integrated Photovoltaic (BIPV)

The Building Integrated Photovoltaic (BIPV) concept is to make a dual usage of the building elements. Examples are solar tiles, solar shingles and solar power generating windows. The idea is to embed thin transparent pre-wired active solar cells between two layers of glass. The glass is cut and framed to form windows and wired to produce electrical power. There are three advantages to this approach:

The windows fit nicely with the contour of the building with no visible external solar panels

A saving factor because the windows are used for their original purpose (bringing natural light into the house) and also produce electricity

Easier access to the windows for cleaning and maintenance than the access to roof mounted solar electric power

Solar Transparent Power Producing Windows Technologies

These are the wo major technologies that are used for the construction of solar windows:

Thin-film solar cells. 9% Efficiency from CIGS (Copper Indium Galium Arsenide) is reported (compared to Silicon efficiency of 20%). One of the vendors is Heliovolt

Organic photovoltaic cell (organic is a nicer name to plastic or polymer). Konarka (a Massachusetts based company) announced recently that it enters this technology, cooperating with Arch Aluminum & Glass. The efficiency claimed is 6%. It will take time for the technology to reach maturity and makes its way to the commercial market

Upgrade path for an existing home?

Solar transparent windows focused market is BIPV and zero energy home (mostly a built-to-order sub market).
Therefore, you don't see it yet offered for upgrading existing homes. I expect however to see a penetration into the home improvement market once the technology is mature, standardized and more cost effective
In the building-integrated photovoltaic sub market, a prominent company is Ertex-Solar (Germany)

Innovative Research

The Queensland University of Technology (Australia) cooperated with Dyesol to develop an innovative technology that imitates photosynthesis. The panel is a transparent sandwich of an electrolyte, a layer of the white pigment Titania (Titanium Dioxide), and ruthenium dye (Ruthenium is a white hard metal) inserted between two glass sheets. Sun radiation hitting the dye excites electrons which are absorbed by the Titania to produce electric current. It will take a few years for the new technology to hit the market commercially

MIT researchers embarked on an innovative idea, to exploit the infrared radiation to generate electricity while letting the visible light in. Quoting the researchers words: "The key technology is a photovoltaic cell based on organic molecules, which harnesses the energy of infrared light while allowing visible light to pass through. Coated onto a pane of standard window glass, it could provide power for lights and other devices, and would lower installation costs by taking advantage of existing window structures."