I've set this up two different ways with my G15. I intially set it up like this:

G7-G11 = /target P1-P5 /cast Flash of Light (Pally) Small heal
G13-G18 = /target P1-P5 /cast Holy Light (Pally) Big heal

I would then go off tanking happily on my main and be able to heal each person in the party with just a single button press. This however, required me to keep my healers window in view to see which party member slot needed healing since they aren't in the same order on everyones screen (fixable with addons though). This setup worked very well for me since it was very easy to use and pally's don't have that many heal spells to work with.

In comes the 3.0 talents and that played havoc on how I was healing. Now, before just spamming FoL and HL were fine... really didn't use Holy Shock or whatnot much at all. But with the new pally talents, it became neccessary for me to use more abilities and I honestly didn't have the real estate on my keyboard to start binding more stuff. So, I came up with a plan:

G1-G5 = /target P1-P5
G7 = FoL current Target
G8 = Holy Light current Target
G9 = Holy Shock current target
G10 = Cast a Seal
G11 = Assist MT and Judge target (15% increased casting speed ftw)
G12 = BACON! (or beacon of light for those outside of the in-crowd)

This got me the functionality I needed and left me with plenty of free G-keys for my other characters. Still had the problem of making sure my healers window was in view for the party slots, but I'm pretty used to that now. I've 2-boxed most of the dungeons and raids out there this way. Raid targeting is definitely hard with this setup since you uhhh, can't target anyone outside the group you are in.