There aren't really any "no go" areas in Belfast any more. Although this changes at certain times of the year. It's not advisable to go into heavily loyalist estates for the bonfires around July 12th. There is often a paramilitary presence and strangers are generally not welcome. If you know someone who lives in the area, ask their advice. Parts of Belfast are divided into sectarian mosaics - Republican and Loyalist estates border each other and can be a flashpoint for violence. The riots sparked by the row over the Holy Cross Primary school earlier this year are a good example.
Check with local people if you're not sure of an area. Many of the estates have excellent examples of political murals and consequently also make interesting tourist attractions. If you feel uneasy in a particular area here are a couple of tips for either area.
Republican:
When you wave to a car that has given you right of way or let you cross, give a pseudo salute with a couple of fingers not your whole hand.
Refer to Derry not LondonDerry.
It's not unusual for people to ask you what you're whole name is - surnames can give away you religion - make it clear you are a tourist and you should have no problem
Loyalist:
Wave to cars using your whole hand
Refer to LondonDerry not Derry
Make it clear you are a tourist to anyone who asks you who you are and what you are doing.
If in doubt about whether you should take a picture of something, ask!
For the record I never had any problems in either area, and I was filming, although a couple of people stopped me to talk about the murals, the troubles and to ask what I was recording.