I fix my murine tissues, mounted on Cryojane slides and plain ole Plue Charge
 slides with pure cold aceton. If I did not use acetone as a fixing agent, 
I would not get immunostaining with many of my murine cell surface markers. 
Formalin and gluteraldehyde do not work for this purpose, short fixing in PFA
can work, but requires long overnight incubations at 4C for some antibodies. 
True, the acetone may sacrifice some morphology, but not getting antibodies 
to work because of crosslinking fixing agents, is time and materials wasted,
even with retrieval methods.  

Acetone is considered a fixing agent, (I believe it prepcipitates the
proteins in place, alcohol does this also - John Kiernan, to the rescue on the
exact mechanism of acetone/alcohol for this purpose) and we try to keep the
sections dry! no water added, in fact the sections (even on Cryojane slides)
are air dried BEFORE immersion in 4C actone to insure the antigen stays where it
belongs and then stains.     

Fixative choice for frozen sections is dependent on what you want to see,
morphology, immunostaining, special stains, no matter what cryosectioning 
method or device you use.  There are no hard and fast rules what fixtive is
used, other than ALLOWING the staining procedure to work in your lab.   

Sold on my beloved acetone as an IHC fixative! and I use others for different 
reasons.  

Gayle Callis