Ulimit command only works after su

[McDougall, Marshall (FSH)]

RH ES2.1,  2.4.21-15.ELsmp

I am trying to give our DBA the ability to test his Sybase installation with
various parm changes, one of which is "ulimit -n".  I have added:

Sybase       soft    nofile  3072
Sybase       hard    nofile  4096

to the /etc/security/limits.conf file and that sort of works.  I say sort of
because if I log in as Sybase and execute ulimit -n 2048 I get:

[sybase at fsh1166db02 sybase]$ ulimit -n
1024
[sybase at fsh1166db02 sybase]$ ulimit -n 2048
-bash: ulimit: open files: cannot modify limit: Operation not permitted

Then if I "su - sybase" and do it again, it works:

[sybase at fsh1166db02 sybase]$ su - sybase
Password: 
[sybase at fsh1166db02 sybase]$ ulimit -n 2048
[sybase at fsh1166db02 sybase]$ ulimit -n     
2048

It's not making a lot of sense to me.  Any enlightenment appreciated.

Regards, Marshall