Travel warnings - Indonesia
Whilst traveling in any foreign country you have to be wary of many things. Many international governments have Indonesia listed as a no go zone due to 2001 and 2002 terrorist attacks on Bali or the 2003 bombing of the Australian Embassy in Jakarta.
Although there has been some concern with the rebels in the Aceh province trying to gain independence from the Indonesian government there has been no concern for tourists within Sumatra. You should have more concern hiring a motorbike and hitting an animal on the side of the road like a dog chicken or water buffalo than dying from a terrorist attach in Sumatra.
Below are links to Government Travel warnings for Indonesia. Most of the travel warnings for Indonesia specifically mention large tourist areas in Jakarta, Bali, Salawesi, Ambon as well as Aceh in North Sumatra. Sumatra as a whole does not have specific travel warnings. You have more chances of being killed in an earthquake or a volcano erupting or a car accident than being killed by terrorists.
There has been various problems on the islands of Salawesi and Ambon with tension and clashes between religious and ethnic groups. Although tourists have not been target several bomb attacks have taken places in churches and public places like markets.
Terrorist attacks hit Kuta, Bali in 2003 and 2005 killing several hundred tourists.
In Sumatra Aceh rebel forces still are fighting against the Indonesian Army for independence so can be extremely dangerous depending on where you go. Banda Aceh is still recovering from the Tsunami which hit 26th December 2004. There have been several ceasefires between the Acehenese rebels and the PLI (Indonesian Army).
Here are links to official government warning from several countries:
Australian Government - Issued 8th July 2007
http://www.smartraveller.gov.au/zw-cgi/view/Advice/Indonesia
New Zealand Government - Issued 21st November 2006
http://www.safetravel.govt.nz/countries/indonesia.shtml
Canadian Government - 4th December 2006
http://www.voyage.gc.ca/dest/report-en.asp?country=130000
United States Government - Nov 18th 2005
http://travel.state.gov/travel/cis_pa_tw/tw/tw_918.html
United Kingdom Government - 1st November 2006
UK Government travel warning to Indonesia
Last updated 20th July 2008