I'm not sure what the problem might be, as I can mount CIFS with BusyBox as well as the 'util-linux-ng.tcz' extension on a "plain" TC 3 system (i.e. freshly booted from CD-ROM, without any persistence):
tc@box:~$ tce-load -wi filesystems-$(uname -r) util-linux-ng 2> /dev/null
Downloading: mtd-2.6.33.3-tinycore.tcz
mtd-2.6.33.3-tinycore.tcz: OK
Downloading: filesystems-2.6.33.3-tinycore.tcz
filesystems-2.6.33.3-tinycore.tcz: OK
ncurses.tcz.dep OK
Downloading: ncurses-common.tcz
ncurses-common.tcz: OK
Downloading: ncurses.tcz
ncurses.tcz: OK
Downloading: util-linux-ng.tcz
util-linux-ng.tcz: OK
tc@box:~$ hash -r
tc@box:~$ sudo mkdir /mnt/x-drive
tc@box:~$ which mount
/usr/local/bin/mount
tc@box:~$ sudo busybox mount -t cifs //192.168.178.48/c$ /mnt/x-drive -o user=USER,pass=PW
tc@box:~$ mount | grep x-drive
//192.168.178.48/c$ on /mnt/x-drive type cifs (rw,mand,relatime,unc=\\192.168.178.48\c$,username=USER,
uid=0,noforceuid,gid=0,noforcegid,addr=192.168.178.48,file_mode=0755,dir_mode=0755,rsize=16384,wsize=57344)
tc@box:~$ ls -l /mnt/x-drive/boot.ini
-rwxr-xr-x 1 root root 217 Jun 18 2010 /mnt/x-drive/boot.ini
tc@box:~$ head -3 /mnt/x-drive/boot.ini
[boot loader]
timeout=3
default=multi(0)disk(0)rdisk(0)partition(1)\WINNT
tc@box:~$ sudo umount /mnt/x-drive
tc@box:~$ sudo /usr/local/bin/mount -t cifs //192.168.178.48/c$ /mnt/x-drive -o user=USER,pass=PW
tc@box:~$ mount | grep x-drive
//192.168.178.48/c$ on /mnt/x-drive type cifs (rw,relatime,unc=\\192.168.178.48\c$,username=USER,
uid=0,noforceuid,gid=0,noforcegid,addr=192.168.178.48,file_mode=0755,dir_mode=0755,serverino,rsize=16384,wsize=57344)
tc@box:~$ ls -l /mnt/x-drive/boot.ini
-rwxr-xr-x 1 root root 217 Jun 18 2010 /mnt/x-drive/boot.ini
tc@box:~$ head -3 /mnt/x-drive/boot.ini
[boot loader]
timeout=3
default=multi(0)disk(0)rdisk(0)partition(1)\WINNT
tc@box:~$ sudo umount /mnt/x-drive/
tc@box:~$
The only differences to the OP that I'm aware is the use of a password and an administrative share (i.e. 'c$'). But the latter should not make any difference at all.