Starting with the figures for the worldwide market, we see that both research firms more or less agree with each other on who is doing (relatively) well and who is doing not so well. HP is leader of the pack, followed by Dell, Acer, Lenovo, and Toshiba. The winners and losers are also quite clear: Dell is losing market share fast, while Acer is doing very, very well, probably mostly because of Acer's very successful line of Aspire One netbooks. Remember, though, that profit margins on netbooks are generally lower.
The worldwide figures in detail, from both research firms:
If we narrow ourselves to the US market, we see a slightly different picture. Apple doesn't play a role in the top 5 vendors worldwide, but in the US, its role is much bigger (which is the reason why during keynotes, Apple exclusively uses US figures). Interestingly enough, Gartner and IDC differ greatly when it comes to Apple: whereas the former sees a year-over-year 2.5% increase in sales for Apple, the latter sees a 12.4% decrease in Apple sales year-over-year. For the rest, it's a pretty clear picture: Dell loses, Acer wins. Toshiba is also doing quite well.
The US figures in detail, again from both research firms:
The figures are not directly comparable, technically, since sales of x86 servers are not included in the figures by IDC. While theoretically this could explain the difference in Apple's figures between the two research firms, it doesn't seem too likely. We'll have to wait until Apple publishes its own figures to see who is right on this one.
Overall though, the decline in sales is lower than what the two firms had predicted, so that could be seen as a good thing. The release of Windows 7 on October 22 could also help the market.