Let's get one thing out of the way: despite being only one of the two companies with declining sales within the US, Apple is doing relatively well. The reason for this is obvious: other PC manufacturers are doing well because they sell lots of netbooks and cheap desktops, machines with low profit margins - the average price of a PC has dropped 20% in the 1st quarter of 2009, and this obviously means that if you're selling low-cost machines, you're earning less money, even if you've increased your sales.
For Apple, the situation is different. The company has very fat margins, and doesn't compete in the cheaper markets. So, even though Apple certainly does feel the punch of the economy, with a decline in sales, it doesn't affect the company that much because of the fatter margins. Anyway, let's loo at the figures in a little more detail.
From a worldwide perspective, Apple isn't even in the list of top 5 PC manufacturers. Toshiba holds the fifth spot, according to both IDC and Gartner, with round and about 5.5% of the Q1 2009 marketshare. This probably explains why during the fancy keynotes Steve Jobs always uses the US-only figures, because those look a lot brighter. In the US market, Apple had a 7.4% market share in Q1 2009, with sales down 1.1% from Q1 2008. This means they are number four in the top five PC makers of the US.
It appears that Apple has little to worry about at this point, but remember that these are estimates, not final results.