When a package has a member file which is a directory, and
a later release of the package has that same filename as a
regular file, the upgrade fails trying to unlink the
directory. RPM needs to be a tad smarter about removing the
existing files and directories.

Here's my test:

# rpm -qlp --dump testdirupgrade-1.0-1.i386.rpm
/testfile 1024 931618691  040755 root root 0 0 912 X
# rpm -qlp --dump testdirupgrade-2.0-1.i386.rpm
/testfile 0 931618699 d41d8cd98f00b204e9800998ecf8427e
0100644 root root 0 0 912 X

As you can see, the testdirupgrade-1.0-1 package owns
"/testfile" as a directory, while the testdirupgrade-2.0-1
package owns it as a zero-length file.

Upgrading from directory to file is a problem:

# rpm -i testdirupgrade-1.0-1.i386.rpm
# rpm -U testdirupgrade-2.0-1.i386.rpm
can't unlink /testfile-RPMDELETE: Is a directory

However, upgrading from file to directory is just fine:

# rpm -e testdirupgrade
# rpm -i testdirupgrade-2.0-1.i386.rpm
# rpm -U testdirupgrade-1.0-1.i386.rpm --oldpackage

I've placed the .src.rpm files for these test packages on
up on web for your convenience..
http://www.davideous.com/misc/testdirupgrade-1.0-1.src.rpm
http://www.davideous.com/misc/testdirupgrade-2.0-1.src.rpm