Steps to reproduce:
1. Open the current book's properties with ALT+ENTER or File>Properties. 
2. Change the fancy date format. For example, both ISO and European cause a crash (other formats are assumed to crash also, but they weren't tested).
3. 


Stack trace:
In unknown file:
   ?:  0* [apply-cb]
In /usr/share/gnucash/guile-modules/gnucash/gnome-utils.scm:
  29:  1* [gnc:options-scm->kvp #<procedure dispatch #> # # ...]
In /usr/share/gnucash/scm/options.scm:
1471:  2  [scm->kvp #<gw:wcp <gnc:kvp-frame*> 0x9cc6af8> ("options")]
In unknown file:
    ...
   ?:  3  [hash-for-each #<procedure #f (section hash)> #(() () (#) () ...)]
In /usr/share/guile/site/slib/hashtab.scm:
 120:  4  (do ((i (+ -1 #) (+ -1 i))) ((negative? i)) ...)
 122:  5* [alist-for-each #<procedure #f (section hash)> (("Business" . #))]
In /usr/share/guile/site/slib/alist.scm:
 117:  6  [for-each #<procedure #f (pair)> (("Business" . #(# # # ...)))]
In unknown file:
   ?:  7* [#<procedure #f (pair)> ("Business" . #((#) (#) (#) ...))]
In /usr/share/guile/site/slib/alist.scm:
 117:  8* [#<procedure #f (section hash)> "Business" #((#) (#) (#) () ...)]
In /usr/share/gnucash/scm/options.scm:
1299:  9  [section-for-each #((#) (#) (#) () ...) #<procedure #f (option)>]
In unknown file:
    ...
   ?: 10  [hash-for-each #<procedure #f (name option)> #((#) (#) (#) () ...)]
In /usr/share/guile/site/slib/hashtab.scm:
 120: 11  (do ((i (+ -1 #) (+ -1 i))) ((negative? i)) ...)
 122: 12* [alist-for-each #<procedure #f (name option)> (("Fancy Date Format" . #))]
In /usr/share/guile/site/slib/alist.scm:
 117: 13  [for-each #<procedure #f (pair)> (("Fancy Date Format" . #))]
In unknown file:
   ?: 14* [#<procedure #f (pair)> ("Fancy Date Format" . #)]
In /usr/share/guile/site/slib/alist.scm:
 117: 15* [#<procedure #f #> "Fancy Date Format" #]
In /usr/share/gnucash/scm/options.scm:
1295: 16  [#<procedure #f (option)> #("Business" "Fancy Date Format" "g" ...)]
    ...
1359: 17  [#<procedure #f #> # #]
1213: 18* [gnc:kvp-frame-set-slot-path # {"us"} #]

/usr/share/gnucash/scm/options.scm:1213:8: In procedure exact? in expression (gnc:kvp-frame-set-slot-path f (symbol->string #) ...):
/usr/share/gnucash/scm/options.scm:1213:8: Wrong type argument in position 1: "us"

Other information:
gnucash-2.0.1-6.fc6 on Fedora Core 6
guile-1.8.0-8.20060831cvs