I really don't think anyone out there has suitcase bombs -- other than the governements we know about (including ours). The fact the the U.S. has nuclear weapons is plenty of reason for worry -- but we have survived it to date. If Bin Laden or any terrorist group loosely related to him had nukes, don't you think they would have used them by now, given what they did with box cutters?
About risks of Russian or Pakistan nukes falling into Bin Laden like hands:
1) Given that Russia is one of the secondary targets I think they will probably increase care on this issue. One extremely stupid thing the U.S. government has done since Bush came to power is end the program we used to have to buy surplus Russian nukes they could no longer guard adequately.There were actually some legit critcicisms of the existing program. (The nukes weren't be bought fast enough, and we weren't being careful to get all of them.) But these criticisms sure weren't justification for ending the program and replacing it with nothing. (This may be the single greatest justification for the "you shoulda voted for Gore instead of Nader" bunch.
2) As to Pakistan, as long as the current government stays in power, or if any governemt to the left of it gets into power , then Pakistan nukes probably won't be smuggled into the U.S. and used. But if the current campaign either provokes a coup in a more fundamentalist direction this might happen. Or if it provokes additional sympathy among enough personell with access to nukes then our fears might come true.
I think that most Pakistan coups in the past have had at least tacit U.S. consent, and that the Pakistani government is working pretty hard (and pretty repressively) to prevent the second scenario. So on the whole - well I did say "don't panic", not "don't worry".