In each sprint I have two 'tasks' - one for bugs found in the current sprint (i.e. on unshipped code), and one for issues found in anything else (any shipped release). This helps me keep track of how much time is lost (per developer) fixing bugs.
Any time logged in the latter category is regarded as waste and it's a key target for reduction. Time logged in the former is reviewed for how it can be more closely linked to the features and changes that caused it.
Don't put estimates against bugs, instead try to add that time to the estimates for unit/functional testing against the features you're working on.
Feel free to adapt any model to suit how your team works - there should be a culture of continuous improvment in any Scrum team, and the devs should be able to suggest and try out improvements as they learn Scrum.
