a; that Nvidia still has a IGP business: 7.2 Mio units ( 1 - 2 )
Correct, many people underestimate it. Much of that is the fairly high ASP MCP79 and MCP89 for Socket 775, but quite a bit of the volume (wouldn't be surprised if it was >40%) is the ultra-low-cost low-ASP MCP61 for AMD, I suspect perhaps the single most successful chip NVIDIA has ever made. Volumes for both markets should shrink to practically nothing by the end of 2011.
b; that Nvidia sells a lot of notebook graphic cards: 8.475 Mio units ( 2 - 3) [...]
Also, AFAIK most notebook graphic cards are lowend to mainstream only.
Yes, but what's 'lowend to mainstream'? Today's low-end is tommorow's ultra-low-cost, and today's ultra-low-cost is tommorow's not-good-enough.
My point simply is that Sandy Bridge won't the dead kiss to notebook discrete GPUs you're expecting. Yes, IGP performance is improving faster than the sweet-spot of the marketing, so it's naturally going to be a bigger and bigger part of the market. But there's nothing magical about Sandy Bridge that will make that happen instantly; it's clearly a better product than the Core i5 IGP, though.
NVIDIA's discrete consumer GPU market is certainly shrinking slowly but surely (down to a fairly reasonable plateau I'd expect) and they're losing the IGP market. But that's precisely why they've invested such a disproportionate amount of money into Tegra and Tesla. They've known for a long time that their core markets were at risk even if they had hoped for a much better outcome in chipsets.
Now if Tesla/Tegra do not deliver (and they've falled short of their growth targets for a long time, especially Tegra) then they're in deep trouble long-term. I don't have visibility on Tesla above and beyond anybody else's, but regarding Tegra I think the design wins for T2 that are definitely going to ship at Moto and LG are better than some are still assuming, and their percentage of design wins for Android 3.0 tablets is excellent.
Qualcomm has little chance before QSD8672, which is a great chip but tape-out was delayed a bit too much due to other projects requiring more resources, and TI/ST-E haven't marketed very aggressively in that field while Marvell/Freescale are usually considered to be second-tier. So that's a big opportunity for NV, and Tegra3 is even massively more exciting on the hardware side - with the big risk factor for them being the software, so we'll see if they deliver better there than for T1/T2. If they do (big if), there's a pretty big chance they blow everybody's expectations for Tegra by a lot, and that'd way more than compensate for IGPs.