Any ex-President will always be a target of terrorists for the symbolic value. But, having seen the Secret Service in operation around DC, I don't think there is any terrorist group with the ability to get the President or an ex-President, unless they get very lucky.
I think the next round of terrorism in this country will be focused on less-prepared second-tier cities. Unless the terrorists can get their hands on a nuke (unlikely) theyy're better off hitting soft targets in a place like Wilmington, DE or Des Moines, where the police forces aren't as ready to deal with terrorism as the NYPD or LAPD.
We now how the Obomination in there and they know he has no stones. Bush is irrelevant to them.