umount - device is busy (nautilus sometimes doesn't close filedescriptors)

[nf2 <nf2 scheinwelt at>]

with curlftpfs i sometimes get "umount - device is busy", because 
nautilus doesn't close file-descriptors when trying to render previews 
or doing mime-type detection for plain text files (i guess). that's the 
impression "lsof" gives me. perhaps this problem only occurs on very 
slow file systems.

norbert