Hi Sergey!
Using GetDriveType API directly, always return 1 for network mapped drives in Windows Vista (only running exe as an administrator).
This morning we start to test this more accuratelly and found that directory doesn't work as I described in another message.
Directory is returning False.
So, my workaround isn't working and all solutions in your site also not works.

[UPDATE]
1. Everything works as expected at design time. The problem occurs only at runtime.
2. We tried to do the same in C# and Delphi and the problem occurs in both.
3. If we disable Vista UAC everything works as expected.

Please, take a look at this blog entries...
http://blogs.msdn.com/cjacks/archive/2007/02/19/mapped-network-drives-with-uac-on-windows-vista.aspx
http://support.microsoft.com/?scid=kb%3Ben-us%3B937624&x=17&y=14