last night was my first practice of the sanchin kata, and i was just curious, should i really be this sore?? sensei and i only went through it at about a 5-7 on the ten scale, but still, it seems that i've re-discovered muscles in my back and thighs that haven't hurt in a while...

One

Sore from the use of muscles?--very normal.

I didn't mean to make you sore!LOL

Sanchin is a very hard kata and can develop a variety of muscles. You're supposed to have someone beat on you when you are doing this kata. Are you in Goju Ryu?
All American Goju Karate

You should always be sore after any training otherwise you arent doing enough.

Break those muscles down boy!!

Being that it was your first practice of sanchin, it is perfectly normal that you became sore. It is the same way when you begin weight training.
Soreness, however, is not a direct indication of an effective training program; i.e. being sore doesn't mean it's working, not being sore doesn't mean it's not working.

Once your body adapts to sanchin, the soreness will decrease, as the intensity of your training increases. If the soreness from sanchin continues for many weeks, reduce the intensity a bit, and ease into it.

-Al

since I started this post, I've added sanchin practice to my daily workout, and it doesn't seem to be all that much of a problem, I suppose that the first night it was just that we worked it after 3 hrs. of other work.

Sanchin31- I'm an Isshin-ryu student, which was developed from goju-ryu, and shorin-ryu. My sensei has told me that he believes that sanchin "beatings" have gone too far in the US and should be more checking for stability than flogging the student. Thus, he will kick a bit at the legs, push from the side and rear, but I am not to worry about catching a 4x4 across the back.