Britain is reequipping and retraining troops in four mechanized infantry and armored battalions to serve as light infantry ("light role units" in British parlance). This is for service in Afghanistan, where the roads are increasingly dangerous because of roadside bombs, and the Taliban gunmen can only be reached off the road, in the adjacent hills. This kind of conversion is nothing new.
In Iraq, many American artillery and armor units were temporarily reassigned (after some refresher training) to infantry duties (mainly patrolling.) This has happened before. During World War II, tanks often served with infantry units. When a tank got hit, most of the crew usually survived, and got out of the vehicle uninjured. They were then expected to "fight as infantry", at least until a new tank was available for them or their damaged tank was repaired. Artillerymen keep their infantry skills up to date, and regularly set up defensive positions when they are in the field. In World War II, artillery units sometimes got hit by enemy infantry, or enemy artillery.