Updated Dec 26, 2000
Fellow 3di followers:
There are some highways around the United States, planned or even complete, that would seem like naturals for 3-digit interstatehood. Perhaps even some helpful person has told you "this is rumored to become I-xxx." Here are some highways that, meritorious as they are, have been confirmed, usually by the state's DOT, to be Non-3di's.
Not I-x85 (proposed) Atlanta, GeorgiaGA 500 is the working route number for Atlanta's proposed outer perimeter, a 200-mile loop circumscribing I-285. As of October 1997, there is no interstate number planned for the beltway. In a letter from GDOT: "...The proposed Outer Perimeter neither exists nor has it been designed, so the Department has made no efforts to designate it as an Interstate facility."
The northern and eastern arcs of this will probably be built over the next 10 years or so. The massive Mall of Georgia (second in size only to Minnesota's Mall of America in the U.S.) is slated to be built by 1999 in the small triangle bordered by the Outer Perimeter, I-985 and I-85 in northern Gwinnett County. 
Atlanta might have its own budding Bud Shuster:
"When it comes to talking out of both sides of his mouth, Georgia Department of Transportation Commissioner Wayne Shackelford has few peers. He's so adept at it that both friend and foe give him high marks. They say that Shackelford, unlike his predecessors, is open-minded and committed to reforming the DOT's asphalt-obsessed ways...
See also: http://www.conway.com/loop.htm
Not I-x81 Scranton, PennsylvaniaThis is the new highway (8 miles opened in November 1997), going northeast from the I-81/84 interchange in Scranton (actually Dunmore). The connection to 81 and 84 is not complete, but when it opens, the highway will become part of US 6. Existing Route 6 will be renamed Business Route 6. [Thanks to the Pennsylvania DOT (http://www.padot.com)].
Not I-408 PhoenixThis number popped up in a misc.transport.road posting, but ADOT archives have no record of this number ever being used. "I-408" was reputed to have been proposed 30 years ago for southwest Phoenix.
Not I-559 AlabamaThis is the Red Mountain Expressway in Birmingham. I was sent e-mail indicating that this was secretly I-559, but it seems that just ain't so. Says Alabama DOT: " Red Mountain Expressway is not I-559 and we have no knowledge of an I-559 in Alabama. - Mr. James F. Horsley Division Engineer"
Well, that's a little interesting. He didn't say "There is no I-559"; he said "we have no knowledge of an I-559." Hmmmm..
Not I-255 IllinoisIllinois does have an I-255; it's just that the section north of I-270, to Alton, is state route 255. We can't rule out Illinois slapping an interstate shield on it later (nearly all its freeways are interstates), but there are no plans now.