i also want to add on, that i am a huge advocate of prosumers, having used them in the past - a prosumer's live view function, especially with the live histogram will give a newcomer a huge edge when it comes to dslr later on. i have also said time and time again that even p&s can produce good pictures, let alone prosumer.
but you see, when a photographer grows, some decide that they want to do more, try new things. this does not mean to say that those who choose to stay with p&s and prosumer are inferior, that is not true, a photographer is never ever judged by his camera, but by his photographs. there is a reason why the term is "photographer", not "cameraman". there is no right and wrong, so long as you feel comfortable with your prosumer, and do not feel limited by it, by all means, it is entirely possible based on the type of shots you like to take.. and stay with it and be content, and do not listen to the equipment whores who will tell you to upgrade.
if you think that there is something more you want to do, and what you have now cannot allow you to do it, like what some other members have mentioned above - even more shallow depth of field, low light handheld photography which require fast shutter speeds, the list goes on.. then by all means upgrade, particularly if you know how to maximise your utility from the dslr.