Co-sleeping includes bed-sharing, room-sharing, and using things like co-sleepers and baby hammocks with baby close to mom. Technically what you are asking about is bed-sharing. Its important to make the distinction.
It is universally accepted that roomsharing is safe and reduces the risks of SIDS. However its not all that practical for a breastfeeding mother because you will fall asleep nursing -nature has designed it that way. Also unrestricted access to the breasts at night has been shown to increase breastfeeding success among other things.
Many studies also show that SAFE bedsharing reduces SIDS risks, among other risks. Unsafe bedsharing or worse sleeping with a baby on a couch, chair, or unsafe location is definitly unsafe. It is unsafe to bedshare with an infant if any of the occupants of the bed are unaware that the baby is there (ie placing baby in bed with an already sleeping parent) or do not agree to take responsibility for the baby. It is also to unsafe to bedshare if the person is morbidly obese, suffers from a sleep disorder that prevents waking, or takes medications that cause drowsiness. You should also not bedshare if you smoke, drink alcohol, or use recreational drugs. Persons too young (toddlers, preschoolersl, etc) to take responsibility should also not co-sleep with an infant. The sleep surface must also be baby proofed.
As to how hard it will be to transition a baby to solitary sleep that depends on many factors including when you decide to make the change and the temperament of the baby. Also keep in mind that whether or not a baby has every bedshared once they are out of their crib they are more than likely going to want to sleep with mommy and daddy. Also an infant used to solitary sleep can at any time start to refuse to sleep alone or in their crib even if you have never bedshared.
Baby already instinctively wants to and needs to sleep with you. Babies do rely on mom to regulate their temperture, breathing, and heart-rate. Just because they *can* sleep alone doesn't mean they should.
The bottom line is that safe bedsharing probably reduces the risks of SIDS more than room sharing and is certainly safer than solitary sleep. Unsafe sleep arrangements are unsafe.
Do not borrow trouble. Do what works right now, change it when it doesn't work. Afterall some day you will want your baby to use the toilet but right now you use diapers. One day your child will feed themselves, but you can't force a baby to feed themselves. One day your baby will be able to tell you what she wants in clear english, but for now she can't. Enjoy each stage and deal with each stage.
Why babies should never sleep alone: A review
of the co-sleeping controversy in relation to SIDS,
bedsharing and breast feeding
http://www.nd.edu/~jmckenn1/lab/articles/McKenna_why%20babies%20should%20n.pdf
CO-SLEEPING: YES, NO, SOMETIMES?
http://askdrsears.com/html/7/T071000.asp
Need vs. Habit
http://www.naturalchild.org/tine_thevenin/need_vs_habit.html
Sleeping through the Night
http://www.kathydettwyler.org/detsleepthrough.html
Studies on normal infant sleep
http://kellymom.com/parenting/sleep/sleepstudies.html