This is a very frustrating subject for me because it seems every doctor has a different opinion. I just had a baby January 12th. Planned my entire pregnancy to breastfeed. I flared the end of November 2010 and was diagnosed with UC (officially) at the beginning of December 2010. Big bummer when having a perfect and exciting pregnancy. 
I had to see a different GI doctor when I started this flare because the one I had gone to in the past (for a misdiagnosed "acute" colitis attack) was on vacation. His wife is an OBGYN so that made me feel good about what he told me to do. He put me on Asacol at first but I told him I couldn't afford it. He then put me on Prednisone. He then went on vacation so I was told to call my regular doc to follow up. So then she chose to put me on Lialda and to start tapering off Prednisone since I was pregnant and that I could continue to take the Lialda through my delivery and with breastfeeding. So I was on Prednisone and Lialda in December. Off Prednisone by the first week of January but still taking Lialda. I was fine. Going to the bathroom normal again. I even called a lactation consultant at the hospital who looked Prednisone and Lialda up in a book and told me they were fine while breastfeeding... Prednisone as long as it isn't a high dosage. I had a very stressful false labor for 3 days before going into active labor... Got sent home from the hospital twice with contractions that were 5 minutes apart, getting no sleep for 3 days straight. When I finally got admitted for labor I was told by the nurse that I couldn't take my Lialda unless they got me a new prescription from the doctors at the hospital. So I just didn't take it the 3 days I was in the hospital. When I started having the false labor during the previous days I started flaring up again. I started taking the Lialda again when I got home on he 14th. At home on the 15th I literally went in my pants on my couch and as I was trying to run to the bathroom, and had no control over it. Bloody and disgusting. It was a Saturday so I had to call the GI doc that was on call that weekend. He told me to stop the Lialda and start 20mg Prednisone and stop breastfeeding. To pump to keep my milk coming in but to throw it out while on meds. Not really sure why he said stop the Lialda. But to sum it all up... I just stopped breastfeeding entirely. I was so depressed that I had to deal with that during my labor and at home with my husband and baby that for the sake of my sanity we just decided to put him on formula. I didn't even have a pump and I was already so tired and stressed about the fact that I might sometimes have to hurry and put my baby in his crib just to make it to the toilet. And to pump every few hours on top of that... I just broke down and took the less stressful option. Although the formula price is stressful as well!
To me, it is just very frustrating because during the course of this entire thing for the last couple of months, I have been told so many different things from 3 different GI doctors and nurses. Which one is right?! It just seems UC is a giant guessing game and we are all their test dummies. Very frustrating.
So I really don't have any advice about taking your meds while breastfeeding. Except try your darndest to do it and try to get as much info as possible. Because I am bummed it didn't work out like I had planned. Hopefully you get better medical advice than I did. Good luck with your pregnacy!