Oh yes,there is a 2nd series to come.
It was so well received tha they started filming No2 practically as soon as No1 was finished showing.
This is from Wikipedia:~
On 12 October 2010 Laura Mackie, the ITV Director of Drama Commissioning, confirmed that the drama will return for a second series in 2011:
We're delighted with the audience response to Downton Abbey and the positive critical reaction. We're extremely proud to have commissioned a series which has clearly captivated ITV1 viewers. Consequently, we're thrilled to be announcing the recommission of a new series for 2011 which will allow us to spend more time with the Crawley family and their servants
The second series will be composed of eight episodes. Filming begins in March 2011. The second series will air from Autumn 2011, and will be followed by a Christmas special later in the year