Is there a way to securely erase an SSD on my MacBook Air?
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.  

http://macintoshhowto.com/osx/how-to-make-an-os-x-lion-usb-thumb-drive.html

You can also make a USB Bootable Recovery Disk as explained here by Apple:

http://support.apple.com/kb/DL1433

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:

https://discussions.apple.com/docs/DOC-3251

See the part where it says 


  How to Zero erase and install OS X


I hope that helps you out.

Updated

The guy at the bottom of this post:

http://forums.macrumors.com/showthread.php?t=1267158

points out a work around to securely erase the SSD drive in the MacBook Air through encryption.

This is what he says:


  cmace127
  
  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 FileVault 2 so that you don't have to worry about this again.