Can I wear a back brace without it harming my pacemaker?
It's not going to hurt your pacemaker (nothing will that doesn't kill you first) but, depending on the style and coverage, any straps may be uncomfortable.
Anything that will not put pressure on the pacemaker's area should be fine.
You know you're wired when...
Your ICD has a better memory than you.
I can bike a 40-50 tour with no trouble.