Okay there are a lot of misconceptions about this scene so I thought I would clear some up for some people so they are not so confused when the new season airs. Also I have a great suggestion which many may not have thought of!
1. Walt is not in New Hampshire. He is there using a fake ID and driver's license. When the waitress realizes he's from NH she says " Your a long way from home" and Walt replies with how many hours (roughly the distance between NH and NM)
2. Everyone knows Walt buys the big machine gun but just to be clear he does not need that kind of gun to kill Skyler, Saul or Jesse.
Now one thing I thought of. Walt makes the "52" out of his bacon which Skyler does for him every year on his birthday (this means he is on the run or she is dead maybe but that's unclear for now) but the waitress tells him his meal is free with a valid ID.
So, either his fake NH ID has the same birth date on it, the waitress just didn't notice the fake ID's birth date was different from the current date, Walt was celebrating his fake ID's birthday to get a free meal (which he didn't want), Walt showed her his real ID and now that waitress can ID him, or Breaking Bad has made a serious continuation error which I highly doubt.
My thinking is that Walt showed her his true ID and left her the $100 tip in order to distract her from remembering the fact that he lied about where he was from to her. But is the DEA comes sniffing she is definitly going to remember the man who gave her such a large tip.
Anyways, I haven't rewatched the episode so I might have missed something on that part so suggestions are welcome.
No the waitress deliberetely checked the ID to verify the birthdate. Furthermore Walt didn't show him his real ID because the waitress notices he's from New Hampshire. Thats the whole reason she says 'long way from home'. So dont read too much into this. Bottom line: Its his fake ID with a birthdate on that particular day. His fake ID might share his real ID or he could have even asked the guy that hooked up the new identities to make his birthday corelate with any of his known aliases, which would be first.
Werner Heisenberg - December 5
Walt Whitman - May 31
Pablo Escobar - December 1
He did say there was a good science musem in New Hampshire. Maybe he goes there, sees a good display of the original Heisenberg and decides to take his birthdate.
PS: Lots of rich folk leave a $100 bill after eating because its easier and faster. $100 is not going to alert any DEA.
I still HIGHLY doubt he ever travels to New Hampshire (within the duration of the Breaking Bad events). So Walt must have gotten a fake ID with an identical birthdate because a) Skyler only made his bacon that way on his birthday and b) why would be ruin something sentimental like that just to fake his fake ID birthdate which he didn't want to fully admit to (showing the waitress the ID) in the first place.
As for the $100, you are right in that the DEA wouldn't normally be drawn to such a person but if the DEA were snooping around there it could tip them off especially if the waitress remembers he didn't want to show his ID either. But it could also lead to nothing too.
The camera also pauses for a long time on the person next to Walt at the dinner counter. The guy looks like he could be undercover or something but this could be a misdirect made to make you think a little paranoid like Walt probably was at the time.
As for the date of his death, somebody on this site mentioned the book Hank gives Walt Jr. and how it mentions Pablo Escobar dying the day after his birthday. So far that is the best prediction for an "end date".