Microsoft has released an update that improves the performance of 3G WWAN data cards for Windows Vista. The update does not appear to be offered currently via Windows Update, and with the recent demise of AutoPatcher, the most direct route to install is for users to download the update manually, and keep a copy for reinstallation.
Specifically, the update improves the auto-speed detection that has been added in Windows Vista. Microsoft notes that this is a new feature in Vista that optimizes network connections for their intended throughput. However, because of the varying nature of 3G connections, Vista at times will underrate the network performance, causing a decrease in overall performance than with a Windows XP system.
This update is also expected to be included in Windows Vista SP1, due out early next year. Both current downloads, linked below, require Windows Genuine Advantage, so it is best to open the links below using Internet Explorer.