My son was also always a good sleeper and all of the sudden at 10 mos. he started getting up a couple of times a night or gettting up for the day at 4am. He is now 15mos. and has settled back into a much better sleep routine ( he still wakes up about 4am some days, but like you at those times I put him in bed with us - fo rthe last hour or so).
There are so many things that could have changed. He could be teething again- this about time for the first set of molars - maybe the pain is waking him up - for that we use Motrin (it lasts 8 hours). He could be hungry - we started a "night-night" snack - just a cookie or two with some milk or juice - that seemed to help get him through the night longer.
As for the co sleeping thing - we just made up a rule that if he gets up after 3:30am he goes in bed with us - we both have to get up about 5am and he will go back to sleep in bed with us where he might not if left alone. I know there are many schools of thought about co sleepign and i agree that it can't go on forever, but if it is not a ightly thing then I don't think of it as a mhabit I think of it as just another trick we use to get through the day/night.
Good luck - I hope it is just a phase for you as it was for us!!