They're not mutually exclusive; even my Atom N270 netbook has SSE3. Not saying it will have AltiVec, but until we actually hear the specs I'll keep my fingers crossed.
Seriously, I doubt it. The N270 is a full blown CPU, with all the die space used on processor-only components.
http://ark.intel.com/products/36331/Intel-Atom-Processor-N270-(512K-Cache-1_60-GHz-533-MHz-FSB)
If it weren't single core and specifically targeting the low power envelope applications, it would be just as at home in a modest desktop system as it is in a netbook. Latest iterations are multi-core, 64-bit and include SSE3 (and even 4) support, because without it, they'd would be useless for the modern x86 object code it is intended to run. Developers stopped using the x87 floating point unit years ago.
PowerPC stopped being a desktop architecture a long time ago now and most embedded uses of it don't even require floating point, let alone vector units. However, basic floating point is a part of the Power specification that has been around since the start, so that will be included. AltiVec is an extension for multimedia/DSP and is only included in higher-end parts with a correspondingly higher cost.
I strongly doubt that a PPC based netbook intended to be affordable will be using Altivec enabled processors. It would be nice, sure, but if I were a gambling man, I'd bet it being a SoC, rather than an AltiVec compatible CPU + separate video/audio/io hardware.