As pointed out, Secure Empty Trash invokes the Locum process, which is a wrapper/placeholder allowing "srm" (secure remove) and other utilities to overwrite or remove files they normally might not have permission to access.
Under normal circumstances, Secure Empty Trash may take a very long time to finish, so long that it may appear to "hang" when it really hasn't. Whenever securely removing large amounts of data, run Activity Monitor with All Processes ordered by CPU usage, and wait 15 minutes to several hours for Locum or srm to finish; if you interrupt Locum by restarting, canceling, or force-quitting, you could leave your HD in an indeterminate state, so you'll need to boot from the Install DVD to repair the HD, not just the permissions.
Under the worst circumstances, Leopard can encounter a permissions problem affecting Locum that Disk Utility cannot repair. Here's how to diagnose and fix it:
1. After verifying that the trash can is completely empty, use TextEdit to save a tiny, one-character text file named "Foo" to the startup drive, then quit Text Edit and drag "Foo" to the trash. While the Activity Monitor window is showing All Processes ordered by CPU Usage, do a Secure Empty Trash and time how long the Locum process takes to complete. If it takes less than a minute or so, it's running normally; otherwise, proceed to step 2.
2. Launch Disk Utility, select the startup volume, and repair permissions. When done, if you see more than one "Warning: SUID file ... has changed an could not be repaired" message, then you need to proceed to step 3; otherwise, step 3 won't help you. In particular, if you see "Locum" in any of the SetUID warnings, then step 3 is very likely to fix the problem.
3. Re-apply the Mac OS X 10.5.1 Updater by downloading it from:
http://www.apple.com/support/downloads/macosx1051update.html
After running the updater again and rebooting, repeat steps 1 and 2 to see if the problem has been fixed.
Anecdotal testimony notwithstanding, Secure Empty Trash has always taken much longer than a normal Empty Trash because it physically overwrites the disk space several (usually 7 to 35) times. Emptying the trash insecurely is far faster because it doesn't take the time to overwrite the file.
However, the extra time for secure removal may only be noticeable in extreme situations. Overwriting large or numerous files can take significantly longer than removing fewer, smaller files. Securely removing files from external drives usually takes longer than from internal drives. Mac HFS+ file systems can be overwritten faster than others. Issues with Time Machine and Locum do increase the amount of time necessary to securely remove files, but in my experience, only marginally when there aren't permissions problems.