TC10284, there is a project to port Linux to old Cisco boxen but I'm not sure how far this progressed. By old, I mean 4000 series.
The reason why you see alternate firmwares for many SOHO devices is because they run Linux and other GPLed software, which requires the manufacturer to give away that source code. It is dramatically easier to produce a third-party firmware when you can base most of it on the source code provided by the vendor.
There is limited value to trying to produce a third-party OS image for Cisco routers and switches. For the devices that are software based, you can get a lot better performance for your buck out of a thoughtfully specced PC - the reason you buy the Cisco device is the whole solution, of which their software is a critical piece. If you're going to roll your own and run Linux anyway, why bother with their hardware? For the devices that are hardware based, the magic hardware that does the heavy lifting is proprietary and undocumented, so you're going to have to do a steep amount of reverse engineering work. If you really truly have the manpower and cluepower to reverse engineer Cisco's hardware chipsets, you should apply that instead to the problem of building your own solution to sell!