I too have back problems and I'm TTC (trying to conceive -- there is a board here for that, too!). I had back surgery Dec. 19, but had gone off the pill in September. Of course, we didn't try while I was on the pain meds and whatnot before and after surgery, but as soon as I got off the percoset and flexeril we were back at it.
I'm now doing PT (and ultrasound and e-stim, which I only do in the first 2 weeks of the cycle when I'm not ovulating) and have been doing some research into this as well. My PT told me that in addition to having had 2 herniated discs (hence the laminotomy and partial discectomy at L4/5 and L5/S1) I also have problems with my sacroiliac joint (which is now the cause of most of my pain).
It turns out that many pregnant women have back pain due to SI problems because the pelvis is prepping itself for the birth (loosening up and such). Also, hormones seem to play a big role in back problems during pregnancy, too. (Remember, I've just done some basic web searching... I'm not offering any sort of medical advice.)
For me, I figure the ends justify the means (meaning that I'm ok with additional back pain for 9 months) -- I've had back pain for several years, so what's a few more more months??
My surgeon basically had no comment when I told him I was TTC, my PT (and two different PTAs) also didn't say anything contradictory. My OB also wasn't concerned either. So, I guess I'm good to go!
I guess that if I was you I would go to your OB/GYN and get a pre-pregnancy check-up, where you can discuss all sorts of things, like taking folic acid, nutrition, other health concerns, as well as your back problems.
Good luck!!