pals who you think might be slightly killed, you should phone and tell them, and ask them how they feel about it, that you know it can't always be easy to hear and then drop the subject entirely.
but otherwise a card is sometimes the only way. nothing worse than opening up a big fat 'guess what' email and seeing a scan pic first thing in the morning if you've just got your period for the umpteenth time...
what i'd probably say, because this TOTALLY defuses the situation at least for the ttcers or people who've lost, is 'we've been trying for some time and so we're relieved and proud etc' even if it's not true.
for some reason the minute i found out that people hadn't had an easy time it was super-easy to celebrate their success. whereas the blithe 'guess whatters' of the world, you just kinda think 'wtf is wrong with me here?' pals who i've discussed this with (single, gay and unlikely to have kids etc) have said it's much easier to know a little bit of that personal background too, jsut makes it seem a little less 'lookit us we're so lucky (and we're not bothering to ask about you)'.