First reports of the third-party 9-cell batteries for the 10.2″ MSI Wind U100 are coming in. Wired Magazine mentioned a thread over at the MSIWind.net forums where somebody ordered one off of ebay.
They mentioned that currently, you are looking at over 4 hours with a standard 6-cell battery in the MSI U100. An MSIWind.net forums user picked up a 9-cell in the UK off of ebay UK, which cost around Â£70 ($120 USD). With WiFi switched on, they were getting over 7 hours of battery life (under Windows XP Home), with some OS X users getting just under 7 hours. These are initial charges, so the numbers should change after a few charge / discharge cycles.
A few of these have popped up in the US – Amazon has a 7800mAh 9-cell battery from Lion Battery that is up for pre-order (and pricier than the one mentioned above).