Can't wait for Gnome3 to the portage? The below steps a basically what I completed to upgrade gnome-light.
Follow these "simple" steps:
edit $HOME/.config/autostart/compiz.desktop and change "X-GNOME-Autostart-enabled=false" to "X-GNOME-Autostart-enabled=false". Compiz is NOT compatible with Gnome3 and will just prevent you from logining in.
edit /usr/portage/profiles/package.mask and comment out the following. (This will need to be done every time you sync.)
>=gnome-base/gdm-2.26
>=x11-wm/metacity-2.34
>=media-gfx/gthumb-2.13
>=gnome-extra/nautilus-open-terminal-0.19
>=x11-misc/notification-daemon-0.7
Add the following to /etc/portage/package.keywords
# Gnome3-light
app-arch/file-roller
app-editors/gedit
app-text/gnome-doc-utils
app-text/poppler
dev-lang/vala
dev-libs/atk
dev-libs/gjs
dev-libs/glib
dev-libs/gobject-introspection
dev-libs/json-glib
dev-libs/libgweather
dev-libs/libunique
dev-libs/nss
dev-python/pygobject
dev-util/gtk-doc-am
gnome-base/dconf
gnome-base/gdm
gnome-base/gnome-applets
gnome-base/gnome-control-center
gnome-base/gnome-core-libs
gnome-base/gnome-desktop
gnome-base/gnome-fallback
gnome-base/gnome-keyring
gnome-base/gnome-light
gnome-base/gnome-menus
gnome-base/gnome-panel
gnome-base/gnome-session
gnome-base/gnome-settings-daemon
gnome-base/gnome-shell
gnome-base/gsettings-desktop-schem
gnome-base/gvfs
gnome-base/libgnome-keyring
gnome-base/libgnomekbd
gnome-base/nautilus
gnome-extra/evolution-data-server
gnome-extra/gconf-editor
gnome-extra/gnome-media
gnome-extra/gnome-power-manager
gnome-extra/gnome-screensaver
gnome-extra/gnome-shell-extensions
gnome-extra/gnome-tweak-tool
gnome-extra/gnome-user-share
gnome-extra/gnome-utils
gnome-extra/nm-applet
gnome-extra/polkit-gnome
gnome-extra/zenity
media-fonts/cantarell
media-gfx/eog
media-libs/clutter
media-libs/clutter-gst
media-libs/clutter-gtk
media-libs/gst-plugins-base
media-libs/gstreamer
media-libs/libcanberra
media-libs/libexif
media-plugins/gst-plugins-*
media-plugins/gst-plugins-ogg
media-plugins/gst-plugins-pango
media-plugins/gst-plugins-theora
media-video/cheese
media-video/gnome-video-effects
net-dialup/ppp
net-im/pidgin
net-im/telepathy-logger
net-libs/libnice
net-libs/libsocialweb
net-libs/libsoup
net-libs/rest
net-libs/telepathy-glib **
net-libs/xulrunner
net-misc/networkmanager
net-print/cups
net-print/cups-pk-helper
net-voip/telepathy-haze
net-wireless/gnome-bluetooth
sys-apps/accountsservice
sys-apps/gnome-disk-utility
sys-auth/polkit
sys-fs/udev
www-client/epiphany
www-client/firefox
x11-libs/gdk-pixbuf
x11-libs/gnome-pty-helper
x11-libs/gtk+
x11-libs/libwnck
x11-libs/libxklavier
x11-libs/mx
x11-libs/pango
x11-libs/vte
x11-terms/gnome-terminal
x11-themes/gnome-icon-theme
x11-themes/gnome-icon-theme-extras
x11-themes/gnome-icon-theme-symbol
x11-themes/gnome-themes-standard
x11-wm/metacity
x11-wm/mutter
# Gnome3 Empathy
net-im/empathy
app-misc/geoclue
dev-libs/folks
media-libs/libchamplain
net-libs/telepathy-farsight
# Gnome Fallback
dev-lang/lua
app-cdr/brasero
app-crypt/seahorse
app-text/evince
dev-libs/gmime
dev-libs/libburn
dev-libs/libisofs
dev-libs/totem-pl-parser
gnome-base/gnome-core-apps
gnome-extra/gnome-user-docs
gnome-extra/yelp
gnome-extra/yelp-xsl
media-libs/quvi
media-libs/taglib
media-video/totem
net-libs/farsight2
net-libs/libsoup-gnome
execute 'echo "-introspection" >> /etc/portage/profile/use.mask
OK, now 'emerge layman'. Once complete, execute 'echo "source /var/lib/layman/make.conf" >> /etc/make.conf'. Now execute 'layman -a gnome'
This is where the fun begins.. this is NOT flawless, but hopefully the errors will be easy enough to remedy.
eg. You may need to unmerge gnome-themes-extras gnome-themes gdm gnome-panel gnome-netstatus to prevent blocked packages.
Now just run 'emerge -puvND world' if you are happy with the result, remove the 'p' switch.
To checkout the newest upstream, run layman -S.
You can always check what has been updated and when by going here
If you want to have legacy menu's then emerge gnome-fallback. This will install more apps that you probably don't use that are "standard" in gnome.