Check out this handy little USB external harddrive enclosure, the Zalman VE-300.
The Zalman VE-300 can be mounted as a USB Harddrive and any ISO stored in the _iso directory on the drive can be directly mounted as an optical device.
Recently used the Zalman to do a new ESXi 5.1 install and it worked great. Just connect it up, select the ESXi 5.1 ISO, boot from CD-ROM device, and away it goes.
The enclosure itself can be had for around $50. The enclosure comes with a case, a screw driver, and a USB cable; just add a 2.5″ hard drive of whatever size you desire and you have a handy portable data CD/DVD library. I picked mine up from NewEgg.com and added a 500GB drive.