I'm releasing a DVD that features popular models from other companies.
This is a commercial release that will be available in retail.
Is it legal for me to mention the names of the companies these models
have worked for on the back of my DVD's box art, in the text? Do I
need to get permission first to use each companies name? Or would giving
copyright notices for each company be good enough (for example: 2 of my
models posed for Gallery magazine, so would saying "Gallery magazine is a 
trademark of Gallery, Inc." at the end of the text)?

The most risk averse course is for you to get permission since I infer
that you might be a competitor of the other companies whose names you
wish to use on your DVD?  If that's the case, they might more readily
object since that fact more immediately creates a likelihood of
confusion, which is the touchstone for allegations of trademark
infringement.  In other words, Gallery magazine might object to your
using their name since a consumer, looking at your DVD and seeing
their name, might be confused into believing your DVD was somehow
affiliated with Gallery and that you were trying to capitalize on the
strength of their name (trademark).

If you choose not to obtain permission, you might be OK as long as you
observe all the strictures of what is called the "nominative fair use"
defense:

?First, the product or service in question must be one not readily
identifiable without use of the [competitor's] trademark; second, only
so much of the mark or marks may be used as is reasonably necessary to
identify the product or service; and third, the user must do nothing
that would, in conjunction with the mark, suggest sponsorship or
endorsement by the trademark holder [competitor].?

See Brother v. Jardine, (Ninth Circuit,
2003)(http://caselaw.lp.findlaw.com/data2/circs/9th/0157095p.pdf)

So, you can use another company's marks to identify that other company's
models as long as you follow the rules and avoid a likelihood of
confusion.  Using a disclaimer like you mentioned is a good idea, but
a disclaimer alone will not save you if you create a likelihood of
confusion by using the other companies' marks.