Getting UTF-8 to work with guile is a bit of a stretch as guile doesn't have any real encoding or UTF-8 support to speak of, but I was able to get at least some basic stuff working by using the Unicode Manipulation routines which are part of the Guile-Glib module.

This requires that you have your LD_LIBRARY_PATH and GUILE_LOAD_PATH set properly so that it can load the glib libraries. This didn't work for me out of the box with guile, but Ubuntu's guile-gnome0-glib package provides a script called guile-gnome-0 for setting these values for you and running guile. If you just run guile as is...

ian@laptop:~# guile
guile> (use-modules (gnome glib))

Backtrace:
In standard input:
   1: 0* (use-modules (gnome glib))
   1: 1  (eval-case (# # *unspecified*) (else #))
   1: 2  (begin (process-use-modules (list (list #))) *unspecified*)
In unknown file:
   ?: 3* [process-use-modules (((gnome glib)))]

<unnamed port>: In procedure process-use-modules in expression (process-use-modules (list #)):
<unnamed port>: no code for module (gnome glib)
ABORT: (misc-error)
guile>

Anyway you can run guile with the right paths on Ubuntu by running guile-gnome-0. After getting that running I made use of the Guile-Glib module to do some operations of utf8 data. I did this from a utf8 console. Beware. The terminal was not to friendly with me about things like arrow keys and backspaces over multibyte characters etc.

guile> (use-modules (gnome glib))
WARNING: (gnome gw generics): imported module (gnome gobject generics) overrides core binding `connect'
guile> (string-length "テスト")
9
guile> (g-utf8-strlen "テスト" -1)
3
guile> (g-utf8-validate "テスト" -1 #f)
#t
guile> (g-unichar-validate (g-utf8-get-char "手"))
#t