I don't know... I think it's quite sweet to get something dad-related. I mean, he's been looking forward to being a dad for years (spent a long time TTC), so it's the baby he's excited about at the moment.
I'm in a similar position, DH birthday is in May, baby due in July. I don't know whether to have his present be something big like a baby carrier (would be specifically for him as I want to get myself a more hippyish sling that I doubt he'd want to wear regularly), or to get him something hobby related as a main present and then give him a little thing like a bib or vest with 'I love daddy' on it as a small novelty.
I think there is a children's book or two about daddies... hmm like this one or I could go for a parenting book like this one as he's already got the Pregnancy for Men book by the same author and seems to like it.
If I go for something baby related, I'd make sure it was something especially for dad and baby, rather than just use it as an excuse to spend DH's birthday budget on baby stuff... however tempting... 
.. yeah, I thought you'd like this bundle of muslin cloths and some nipple cream dear... 
I understand that the point of view that we're more than parents so people suggesting non-baby related stuff, but it's also a huge part of what's going on his life at the moment and he's really looking forward to being a 'dad' at last. I think it depends on the man involved whether you think he'd appreciate something dad-related or would rather have something that reflects his pre-parenthood interests to remind him that not everything has to change, that he's still himself as well as a dad-to-be.
Anyway, I'm quite interested in any ideas others might have as I'm undecided.