That is about the truth - the problem comes in when attempting to use the "nVidia" shaders with ATI cards.

A lot of the ones in the library which nVidia provides will have problems with ATI video cards. However, if you learn what is actually going on and write the shaders with what is needed - you will find that the majority of them will run best on ATI cards (one of the reasons nVidia cards perform so piss poor with HL2). There are a number of tweaks which are included in the ones provided by nVidia to make them run on their cards which mess it up for cards which actually have full support for the new shaders.

BTW - I do highly recomend ShaderX - very good book for anyone who is planning on using shaders extensively - and chances are really good you will be able to snag one from a discount book store now. Or check the Wordware website - as they often have package deals on books and ShaderX is a very popular title for the package deals.