You've sure got a nice rack....

[megahurts]

I was doing some searching trying to determine how the moisture from upper trays dripping on lower lights is dealt with.  I still haven't found a straight answer, but I guess that a gfci protects you in case of a short situation.  I just hate this kind of thing because I will be leaving this unattended for hours at a time.  I'm not paranoid, just very safety conscious.  Anyway, I encountered this article for building a propagation rack that looks pretty diesel.  I thought I'd share.
 
http://www.nrcs.usda.gov/Internet/FSE_PLANTMATERIALS/publications/mipmctn11120.pdf

 

 

[juanitos]

well the shelves are plywood then he sticks plastic sheet on the shelf as well. The trays probably don't have holes..

 
DO NOT WATER PLANTS ON THE SHELVES. 
Remove propagation containers from the shelf before 
watering. Allow container to drain before placing back on 
shelf. This will minimize the opportunity for water to 
come in contact with the electrical system. 
 

Oh here he says to not water on the shelf... that'd be annoying, who's going to remove all the trays every watering...
 
This is a nice guide for building the shelf atleast. I made one like this but only 4x2 not 4x4 as that's pretty damn big to fit in my garage.

 

[Spork]

That is a pretty nice looking shelf, thanks for sharing. I pull most of my trays for watering because I bottom water them at that stage, but they still leak some residual water which would probably warp/damage the plywood over time. May be a good idea to use vinyl or something to protect them.
 
 
 
 
Edit:
Looks like it would cost a fortune to build one:
 

2"x2"x8' Boards to hold shelves $12 (4 X $2.79):
http://www.homedepot.com/p/203758918
2"x4"x8' Boards for legs and cross beams $25 (9 X 2.78):
http://www.homedepot.com/p/202091220
1/2"x2'x4' Plywood (fancy birch plywood) $73 (5 x $14.50)
http://www.homedepot.com/p/203552995
2x 25-pack of 3" screws $4:
http://www.homedepot.com/p/100348673

50 pack of 6" screws $40: (not a typo, $40 for screws)
http://www.homedepot.com/p/202819719
4x 3" casters $22 (4 x $5.48)

http://www.homedepot.com/p/203672418
 

We're at almost $200 without lights, electrical, reflective material + adhesive.
 
My $0.02 
Much cheaper to buy a used wooden bookshelf unless you can get the above stuff much cheaper or free.
 
You could consider an upside-down shallow plastic tub to protect the lights:
http://www.walmart.com/ip/20699654
If you were really paranoid, you could run PVC pipe to put the wires in, too.