If you have a USB Flash Drive of 8 GB of size, you can make a Lion Installer and use it to securely erase the Drive.
You can also make a USB Bootable Recovery Disk as explained here by Apple:
Once you have done either you can hold down the
OPTION key while restarting your computer and booting from it.
When it loads up, it will give you a few different options, you will need to use the Disk Utility part and highlight your HD and choose the Erase Tab. Under the erase settings will be a security option when you can write the disk to 0 once or even multiple times.
There is a guide on it here:
See the part where it says
How to Zero erase and install OS X
I hope that helps you out.
The guy at the bottom of this post:
points out a work around to securely erase the SSD drive in the MacBook Air through encryption.
This is what he says:
I found a workaround.
Restart the computer and hold option to enter the setup screen. Go
into disk utility and select the drive. Erase the drive using "Mac OS
Extended (Case-sensitive, Journaled, Encrypted). Make a password for
the encryption, it doesn't matter what it is because you won't need
it. Hit "Erase". Now select the volume and the "Erase Free Space" and
"Security Options" buttons should no longer be grayed out. Click and
select your level of security and off you go. I presume "Erase Free
Space" and "Security Options" should do the same thing because you
just erased the drive so all space is considered free. This worked for
me so let me know if it helps.
Let me know if that's works, this should securely erase the drive since it will then be encrypted.
Also, in the future with lion be sure to use File Vault 2 so that you don't have to worry about this again.