I noticed the following line in the semantic-f90.el:

 (modify-syntax-entry ?&  " "  f90-mode-syntax-table)

This causes the & to be considered whitespace. This causes some issues if you cleanup trailing whitespace. 
Suppose you have the following line:

a = 1 + &

In this case after a delete-trailing-whitespace, the & will be deleted, which is incorrect. 

a = 1 +
    1  ! Invalid code

See http://fortranwiki.org/fortran/show/Continuation+lines for some details. 

I think that it can be considered an escape character (if placed at the end of a line). Perhaps when placed at the start of the line (after optional whitespace), it can be considered extra whitespace.