When building your site collection (in SharePoint 2010) using publishing sites, by default, each subsite uses the global navigation of its parent and subsites are visible in navigation . So even when using dropdowns in the global navigation, it is no big deal to change the order of the subsites in the dropdown, for they are visible in the navigation setting by default.
But if you build your site collection using eg. team sites (on SharePoint Server 2010, Foundation is a different story) and still wish to have the same kind of navigation as described above, there's a lot more work to do and everything isn't as obvious.
First of all, of course, you need to activate SharePoint Server Publishing Infrastructure feature in your Site Collection.
Then you need to remember to set each new site to use the navigation of the parent site - which can do in the creation options - plus, if you want the subsites to be visible in the dropdown, you'll need to set that up in the site navigation settings after creating the site.
Doing this, everything seems to be just dandy. The subsites appear in the dropdowns as they should. But what about when you want to change the order of the subsites in the dropdown? You can't see them in the navigation settings of the top level site. You can't even see them in the navigation setting of the parent site for the subsites in the dropdown. The Show subsites is seledted, but...?
Subsites aren't really a part of the global navigation even though you can choose to display them in it. They are a part of the Current navigation sitemap. So, in order to change the order of the subsites in the global navigation, you actually need to change their order in the Current navigation. Which means that in the navigation setting of the parent site you need to select Show subsites in the Current navigation settings:
This will of course display the subsites in the navigation editing and sorting box and allow you to re-organize them. This setting, however, also results in the subsites being displayed in the quicklaunch navigaton. Maybe you want it to be so, maybe you don't. If you don't, then simply de-select the Show subsites checkbox after re-organizing the sites. The order will still be preserved when you click ok, and be applied to the global navigation dropdown.