The Apevia X-Sonic is a very light case, but includes a crap PSU - basically the same design as the original Antec Lanboy. Luckily the crap L&C PSU doesn't add much to the cost of the case - maybe you can dump it for $10. on eBay...
The Antec Super Lanboy is still available - it's light and doesn't make you pay shipping on a crap PSU. Always check Provantage.com for pricing on current Antec models. Often beats them all on bottom line. I see Antec has some in B-Stock for $35. +sh.
This one is very light too:
skyhawk but may not actually be in stock - check by phone before ordering. Should be under 12 pounds net.
And directron.com has a long list of aluminum cases but quite a few are Al and steel, like the Centurions - pays to know your cases before diving in there. Several very cheap ones.
.bh.