The sample is a hotel registration card that Alshehhi filled out:
I found this on a handwriting analysis site. It looks real, but I don't know the original source. (The site appears to be honest. They also have copies of the so-called terrorist suicide notes that we've discussed; I verified that the images they post are identical to the images at the FBI web site.)
The most important thing to notice is that Alshehhi wrote the dates as: "8-26 - 9-2-01". Notice the American date order (month-day-year). This goes against some people's claim that the American ordering of the dates in the anthrax letters suggests that the anthax letters were written by an American, not a Middle Easterner. This hotel registration card confirms that the 9/11 terrorists were sufficiently familiar with our customs that they would write dates in the order we normally use. [Just to jog everyone's memory, the anthrax letters are all dated "09-11-01".]
The second thing to notice is the use of dashes in the dates, instead of the more common slashes. Dashes were also used in the dates in the anthrax letters.
A difference between the dates here and the dates in the anthrax letters is that the month on the anthrax letters is written as "09", while the months on this registration card are written without the initial "0".
I'm no handwriting expert, but this doesn't appear to be the same hand as the anthrax letters. Of course, the writer of the anthrax letters was apparently trying to disguise his handwriting. He printed, and the writing is very neat and deliberate. This sample is written in cursive, and it was scrawled quickly, so they're difficult to compare.
I won't bother reposting the anthrax letter or envelopes, since I don't see a lot of similarity in handwriting between those documents and this sample. The 6 and the 9 look similar, but the 1, 2, and 8 do not. There's no slant in the hotel card sample. If you do want to compare them yourself, you'll probably find it useful to get a graphics program and enlarge the images.