Upon further research

...thanks to an AFM link, SOME airlines will waive this restriction for musical instruments.
Still, flying with tubas is a big deal (maybe I could suggest an ITEA article on it?) Perhaps someone who flys a LOT (Oystein, Pat?) would care to comment??
The following (so far) is what I am finding:
In GENERAL, unless otherwise noted, the limit is 80 inches.
Jet Blue - no additional allowances at all (musical instruments are listed as having to fall under their normal requriements)
Delta - allows up to 120 inches
American - does have an exception, but does not list it (other exceptions are 115 inches, so I am guessing it is similar)
United - allows 115 inches for oversize in general
Southwest - no allowance listed
Midwest - allows up to 115 inches in general
Continental - up to 90 inches NOT subject to oversize fee, 116 inches max oversize limit
ATA - no provision listed (other than stating they are not responsible if they break it)
US Airways - no provision listed
I am going to keep updating this post as I research.
Here is AFM's very helpful page
http://www.afm.org/public/departments/airline_links.php