The question is simply stated in the subject. Again, from which movie
is the quote: "I would walk on glass and eat donkeys for you."

Hello.

The movie was "My Boyfriend's Back" (1993).
http://www.imdb.com/title/tt0107626/

The actor who uttered the line was Matthew Fox, now known from the
television show "Lost."

I ran the quote through a newspaper database available through my
local public libray and came up with:

'It's time to take another look at 1993's "My Boyfriend's Back," a
comedy about a teenager who dies, and then crawls out of his grave to
take his dream date to the prom.
...Matthew Fox of "Party of Five" looks very clean-cut as the dead
guy's rival, Buck Van Patten. Fox wears tennis sweaters and a
letterman's jacket, and woos his steady by saying things like, "I
would walk on glass and eat donkeys for you."'
source: Chicago Sun-Times, June 8, 1998, page 33
http://nl.newsbank.com/nl-search/we/Archives?p_product=CSTB&p_theme=cstb&p_action=search&p_maxdocs=200&s_dispstring=(eat%20donkeys)%20AND%20AND%20date(6/8/1998%20to%206/8/1998)&p_field_date-0=YMD_date&p_params_date-0=date:B,E&p_text_date-0=6/8/1998%20to%206/8/1998)&p_field_advanced-0=&p_text_advanced-0=(%22eat%20donkeys%22)&p_perpage=10&p_sort=YMD_date:D&xcal_useweights=no


A "Lost" fan also mentioned the quote last year on a messageboard
devoted to that show:
 
"... i also like how he said he would eat donkeys for her.. how random
was that??!!"
source: Lost Forum, 4/26/205
http://lost-forum.com/archive/index.php/t-6126.html

-------
search strategy:
newspaper database: "eat donkeys for you"
google: "boyfriend's back" "eat donkeys"

I hope this helps.

Excellent and detailed answer. Research well done, juggler. Thanks. :)