When using latest CFFI via clbuild2 (git://common-lisp.net/projects/cffi/cffi.git), I encountered a problem with :int64 fields. CFFI seems to treat this field as (signed-byte 32). This is with SBCL x86-64 on Ubuntu 10.10:

This is SBCL 1.0.55.1-6548750, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
* (require :cffi)

NIL
* (cffi:defcstruct foo (a :int64)) 

FOO
* (cffi:with-foreign-object (foo 'foo) 
  (setf (cffi:foreign-slot-value foo 'foo 'a) 123456789012345))
; in: CFFI:WITH-FOREIGN-OBJECT (FOO 'FOO)
;     (SETF (CFFI:FOREIGN-SLOT-VALUE FOO 'FOO 'A) 123456789012345)
; --> LET* MULTIPLE-VALUE-BIND LET PROGN CFFI::FOREIGN-SLOT-SET SETF LET* 
; --> MULTIPLE-VALUE-BIND LET PROGN CFFI::MEM-SET CFFI-SYS:%MEM-SET LET SETF 
; ==>
;   (SB-KERNEL:%SET-SIGNED-SAP-REF-32 FOO 0 #:VALUE14)
; caught WARNING:
;   Derived type of #:VALUE14 is
;     (VALUES (INTEGER 123456789012345 123456789012345) &OPTIONAL),
;   conflicting with its asserted type
;     (SIGNED-BYTE 32).
;   See also:
;     The SBCL Manual, Node "Handling of Types"
; compilation unit finished
;   caught 1 WARNING condition

debugger invoked on a SIMPLE-TYPE-ERROR in thread
#<THREAD "initial thread" RUNNING {10029B97C3}>:
  Value of #:VALUE14 in
    (SB-KERNEL:%SET-SIGNED-SAP-REF-32 FOO 0 #:VALUE14)
  is
    123456789012345,
  not a
    (SIGNED-BYTE 32).

Type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [ABORT] Exit debugger, returning to top level.

(SB-C::%COMPILE-TIME-TYPE-ERROR
 (123456789012345)
 (SIGNED-BYTE 32)
 #<unavailable argument>
 ((SB-KERNEL:%SET-SIGNED-SAP-REF-32 FOO 0 #:VALUE14) . #:VALUE14))
0] 0