Let's see, someone said they had accepted a job offer on the spot but then might have another perhaps more preferred job offer given in succeeding weeks and could they renege on their acceptance. I agree with the person who said, essentially, that they accepted the job and should stick to it. I don't know the market they are in, but you don't want to develop a bad reputation, particularly in a smaller, or medium sized market. If you accepted, you should stay with the folks who gave you a job. HP is very into loyalty and integrity. Next time, if possible, try to buy some time and follow up with the potential outstanding firm. Of course, now that you are going to firm 1, you need to do a fabulous job, be enthusiastic, etc. Even if this is not ultimately where you want to be, you want to be in a position that you've earned the respect of these colleagues and built your reputation, can get great references, etc. I spent my 1L summer in a different market from where I work now. It was a medium size firm, but not the sophistication of BIGLAW. Did I want to work there forever NOOOOO. But, I got great experience (accompanying partners to court, drafting summary judgment motions, even working on criminal cases), AND I made terrific contacts who later served as references. Especially when these experiences are hard to come by, you need to grab them and run with them. Remember, every opportunity is an opportunity to build on, even if it is not your end game.
I got a long email from a gentleman who wanted to share some advice on shirt "stays" for collars and other dressing advice; I will send that along next time. He did mention poplin suits for summer. I have to say I am not a fan and would rather stick with the basics. Now, maybe it depends on the market, but I just think they look goofy and kind of grandfatherly and have not seen people in firms I have practiced or worked with wearing them.
One more thing, the other day, I got a call from a lawyer I worked with many years back on a matter. She is now the lawyer on a different matter and we will be working together again. We exchanged needed information and will continue to work together as the weeks progress. The point I wanted to make here is that because we handled our last matter professionally, she knows that I will do my job while being someone who is good to work with. I know that she will be careful, yet firm, but still professional. We can each tell our clients that we have worked together before, which will give them confidence that the matter will go smoothly. This goes to my point of not being an a-hole (unless truly necessary) and being professional. Even in larger legal markets, what goes around comes around. You do need to deal with these people or their firms again and your reputation is always important.