Pop, it's wonderful that you've felt healthy and happy throughout your pregnancy, and haven't had a problem!
Personally, I have a bulging disk in my spine, and have been suffering bad MS for the last few weeks. When I'm feeling well and happy, I'm perfectly happy to stand on the train - and do so on a regular basis considering I commute into London every day! But when I'm feeling particularly awful and faint and can't stand, I feel it particularly unfair that fit business men barge onto the train to take the priority seats, when they should be prioritised for those who need them.
I have always given my seat up for those who needed it more - pregnant women, older people, young kids, those who clearly need a seat (crutches or whatever). It's just basic common decency. All I'm asking is that now, in my days of need, I can get the favour returned! The idea of wearing a badge is so that people realise I am actually pregnant, not just fat (although it's quite obvious now). Oddly, most people I talk to think they're a good idea, meaning that they don't offend people by offering them a seat, which some people claim to be afraid of.
It's not even standing for a 40 minute journey that annoys me - as PastGrace says, it's the being elbowed and shoved around. I find it even more annoying when you're shoved into the middle of a train with nothing to hold onto (I'm particularly small and can't grab the overhead hand holds!). I forgot - about a week ago I was on a really crammed train, and naturally nobody offered me a seat, but a lovely tall man did help me shuffle to the edge of the train so I could at least rest on a door, without just tumbling around in the middle with nothing to hold on to! He made my day.
I think I may start asking on those days when I really need a seat. It isn't that often, after all (normally on my early starts, which are three days a week, and not every day), and some people really just don't pay attention.
I also think its the older people who are much more observant and helpful on this front. My DH was very ill on a tube train a month or so ago; after politely declining a seat an old lady offered him (nobody else bothered!) she said "You look green, please take my seat". He was very grateful; she tottered down the carriage with her walking seat and bugged someone at the other end until they gave her a seat! Wonderful woman :D