According to a report by analysts IDC, notebook sales will outstrip desktop PCs by 2011. The report goes on to predicts that the notebook market will grow by 16.1% year-on-year until 2011, compared to 3.8% for desktops.
The report also goes on to predict that Windows Vista will have only a small positive effect on desktop sales before the downward trend continues.
I think that the predictions in the report are plausible. But I think that there's a reason why - notebooks are less reliable, have a shorter lifespan, are harder to upgrade and expensive to repair compared to desktop PCs. This translates into more being sold.