I tracked down the problem:

in conf.c there is no mmap() routine associated with /dev/vga
anymore because the entry is now declared using dev_tty_init instead
of dev_pc_init. I don't know when this change was made and if it
should be considered as a bug or a feature.

If it's a feature, you have to disable kernel security or to install
the aperture driver to run Xfree86.

By the way, if you want to recompile the aperture driver on -current,
you'll need this patch:

*** lkm.c~	Sun Nov 20 11:23:17 1994
--- lkm.c	Thu Jan 19 21:20:24 1995
***************
*** 14,21 ****
--- 14,26 ----
  /* cdevsw-specific types */
  #define dev_type_read(n)        int n __P((dev_t, struct uio *, int))
  #define dev_type_write(n)       int n __P((dev_t, struct uio *, int))
+ #if NetBSD <= 1994100
  #define dev_type_ioctl(n) \
          int n __P((dev_t, int, caddr_t, int, struct proc *))
+ #else
+ #define dev_type_ioctl(n) \
+         int n __P((dev_t, u_long, caddr_t, int, struct proc *))
+ #endif
  #define dev_type_stop(n)        int n __P((struct tty *, int))
  #define dev_type_reset(n)       int n __P((int))
  #define dev_type_select(n)      int n __P((dev_t, int, struct proc *))
***************
*** 45,55 ****
      return(0);
  }
  
  int
  ap(struct lkm_table *lkmtp, int cmd, int ver)
  {
      DISPATCH(lkmtp, cmd, ver, ap_load, nosys, nosys)
  }
! 
  
      
--- 50,67 ----
      return(0);
  }
  
+ #if NetBSD <= 1994100
  int
  ap(struct lkm_table *lkmtp, int cmd, int ver)
  {
      DISPATCH(lkmtp, cmd, ver, ap_load, nosys, nosys)
  }
! #else
! int
! ap(struct lkm_table *lkmtp, int cmd, int ver)
! {
!     DISPATCH(lkmtp, cmd, ver, ap_load, lkm_nofunc, lkm_nofunc)
! }
! #endif
  
      


					Matthieu