You can only flashback the whole database or flashback some table: you cannot flashback a tablespace or a datafile.
But you can try to restore a tablespace at some point in time in the past with tablespace point in time recovery (TSPITR) but only with database backups: http://download.oracle.com/docs/cd/E11882_01/backup.112/e10642/rcmtspit.htm#BRADV89787.
Edited by: P. Forstmann on 16 févr. 2011 09:00
You could combine the two features flashback database and transport tablespace to in effect flashback a tablespace.
1) flashback database
2) make tablespace read only
3) export metainfo
4) copy datafile
5) recover database to the point before you did flashback database
6) drop tablespace
7) import tablespace again
But the other option would be to do TSPITR as mentioned by others already. Look here for an example: