iGoogle finally offers a way to reorder tabs: just go to google.com/ig/settings and use the arrows displayed next to each tab name in the Content section. This solution is not very elegant, compared to other services like Netvibes, where you can reorder the tabs using drag and drop.
If you're wondering how to access the settings page from iGoogle, click on the small arrow displayed next to the name of the current tab and select "Edit this tab". The settings page is also the place where you can select your location, choose to automatically open links in a new tab, change the name and the layout of a tab or backup the iGoogle page. Oh, and don't forget to click on the barely noticeable Save button every time you change the settings of a tab.