I can't believe I'm living in a society (in the year 2011) where mothers still feel guilty for such a thing!
Absolutely, go for it! I made a decision waaaay back when I was doing my degree in Psychology (specialised in Child Psychology) that even if I decided to become a stay at home parent, I would probably use nurseries or other forms of childcare/play groups from a young age. There is plenty of research that shows that when the nursery is of a good quality, children who attend ultimately do better in school and develop better social skills. (as with most research this does not mean they always outperform children who stay at home - just "more likely". nothing to stress about, in my opinion)
I know of many children who have attended nursery from a much younger age than 15 months and turned out absolutely brilliantly (and many from the age of 4 months).
I'm planning on using a nursery for my child when she's around 4-6 months old for about a day per week, even though I will still be on maternity leave. I think it will be beneficial for her, so long as it's a good quality nursery (e.g. Outstanding Ofsted rating etc.. and money is no object). We'll gradually increase when I return to work. I truly believe it's healthy for children to have a variety of experiences and interactions with many different people, rather than cooped up in the house with mum all day. Mum is there to coach / monitor development, not be the sole source of it.
I just wish mums weren't always giving themselves such a guilt trip! I view good parenting a bit like being a good CEO. The CEO doesn't do all of the work him/herself. A good CEO is adept at selecting the best talent to run their organisation to meet or exceed their business strategy.
My family strategy is to raise a happy, loved, intelligent, and successful child who is surrounded by supportive friends and family. And I'm not afraid of delegating some of the work to people I select to help me achieve that goal.
I've never meet a successful CEO (or indeed a successful parent) who thinks they need to do all of the work him/herself and is afraid to leverage other people effectively to achieve a superior result.
Anyway that's my philosophy and I'm sticking to it ;)