I own a do-it-yourself gardening company, I would like to do some 
news print advertising  next month, and would like to use the phrase
?we are the IKEA of home gardening? is it legal to use another company
name as your tag line?

I'm almost positive that you can. I remember reading years ago about
this and one of the examples they gave as a brand name that had lost
it's branding characteristics because of product popularity was
'fridge'. Although fridge is short for the brand name Frigidaire, by
fridge almost everybody understands that you are refering to any
refrigerator and not necessarily a refrigerator by Frigidaire. When
that happens a 'brand name' looses it's copyright protection, that's
why companies want their brands to be well known but not so popular
that they become a word rather than a brand. So, I believe you should
be able to say "we are the IKEA of..." implying a market position of
some kind.

Eli

There was also that court case concerning Xerox, Kleenex, and
Crayola's names that they lost, giving people free-reign over those
formerly copyrighted names.

IKEA is not generic enough.  You can get in trouble.  Consult a lawyer.

Not being legal advice, I don't think Ikea is in the same realm as
Crayon, Xerox and Kleenex, it is too unique and not generic and common
enough.  I buy "do it yourself" furniture all the time, I've never
called it Ikea, but when I sneeze I grab a kleenex no matter the
brand.  You could try it, but Ikea might not like it.  They'd probably
ask you to stop using it before they sued you though.  If you do try
to get away with it, you should attribute that it is the TM/registered
trademark of the company in your ad.

And to be honest, for as big as they say Ikea supposedly, I never
heard of it until about a year ago, so that might not be the best way
to market your company - potential customers might not have any idea
what it is.