(ET) 12M brake disc

[tbamc <tbamc gbta net>]

My pedal wasn't returning well--without some encouragement from me.  
Lubed several places and that's better, but while under there I noticed 
that the brake disc (not the cast iron set up) was flopping a bit rather 
than solidly locked to the axle.  Appears to have rivets (?) holding 
it.  Outer edge of disc will perhaps move 1/8 of inch, left to right.  
Some in the archives talk of 'floating' discs.  Is this acceptable, or a 
problem that needs fixing?  My manuals don't have much info on this.

Thanks.

Thon