modification of zlib/libpng license, is this legally usable?

[Anders Bergh <anders1@gmail.com>]

Hi

I've modified the zlib/libpng license to be a bit more restrictive for commercial usage.. I would like to get some feedback, and I also wonder if it's legally "usable".

<LICENSE>

Copyright (c) <year> <copyright holders>

This software is provided 'as-is', without any express or implied warranty.
In no event will the authors be held liable for any damages arising from the
use of this software.

Permission is granted to anyone to use this software for any non-commercial
purpose, and to alter it and redistribute it freely, subject to the following
restrictions:

   1. The origin of this software must not be misrepresented; you must not
      claim that you wrote the original software. If you use this software in a
      product, an acknowledgment in the product documentation would be
      appreciated but is not required.

   2. Altered source versions must be plainly marked as such, and must not be
      misrepresented as being the original software.

   3. This notice may not be removed or altered from any source distribution.

For commercial usage these restrictions applies aswell:

   4. You must have written permission by the respective copyright holders
      for using this software.

   5. The usage of this software must be acknowledged somewhere easily
      accessible for the end user, such as the about box or product
      documentation.

</LICENSE>

Anders