This was a weird one. I did a clean install of RC1. I used my Dell driver CD to reinstall drivers. After installing the sound, chipset, and SATA drivers the DVD drive would disappear at the end of each installation. I rebooted and BIOS did not detect it. I had to power off at the end of each installation to get the drive back. I've copied the entire contents of the drive to the hard drive and had no problems. I installed one of the drivers from this copy on the hard drive and had no problems. This is all now working so I probably won't play too much with it but I wanted to put it out there for others to consider.
Drive identified as: TSSTcorp_DVD+-RW_TS-L633B
Also lists D200. I'm wondering if this is the firmware version.
Dell Latitide E5400
More about :dvd drive disappears installing drivers
I installed the Win7 Enterprise RC 1 7100 build on my Dell T3400. I didn't install any additional drivers. After awhile my DVD drive would just disappear. You couldn't access it, it wouldn't be listed anywhere, it wasn't visible in Device Manager and scanning for new hardware didn't find it. The only way to get it back was to reboot the system. The DVD drive is the TSSTcorp DVD+-RW TS-H653F using firmware A02 with a release date of 4/29/2009 on Dell's driver download section.
I was able to resolve this issue by adjusting the Suspend Mode configuration in the BIOS. I changed the default factory setting of S3 to S1. Boot to BIOS, select Power Management, select Suspend Mode and change from S3 to S1. Save changes and reboot.