[Q] su only working from adb on 'rooted' Galaxy Tab 2

I used clockworkmod recovery and the cwm-root-gtab2.zip package to root my Tab 2. su appears to be installed ok, and I can get root permissions by running it from an adb shell:

~ $ adb shell
shell@android:/ $ id
uid=2000(shell) gid=2000(shell) groups=1003(graphics),1004(input),1007(log),1009(m ount),1011(adb),1015(sdcard_rw),1028(sdcard_r),300 1(net_bt_admin),3002(net_bt),3003(inet),3006(net_b w_stats)
shell@android:/ $ su
shell@android:/ # id
uid=0(root) gid=0(root) groups=1003(graphics),1004(input),1007(log),1009(m ount),1011(adb),1015(sdcard_rw),1028(sdcard_r),300 1(net_bt_admin),3002(net_bt),3003(inet),3006(net_b w_stats)
shell@android:/ # ls -la /system/bin/su
-rwsr-sr-x root shell 22364 2008-08-01 12:00 su
shell@android:/ #

So the su binary is present and has setuid bit set on its permissions. But if I run it from a SSH session, or via a terminal emulator on the Tab itself, I get permission error:

u0_a108@android:/ $ su
Permission denied
1|u0_a108@android:/ $


Various root checking apps report similar problems. What am I doing wrong? Other threads suggest steps to recreate the su binary with the correct 6755 permissions, but as far as I can see, everything is already as it should be permissions-wise.
Thanks.