My voice is low enough that except for about the lowest tenor note, I'm perfectly comfortable, and I actually have to go into my head voice (falsetto, or whatever you call it - I've never really learned this stuff!) on the higher notes for a tenor. Alto is almost ALL head voice for me. Which makes me...what? A contralto? I dunno.
I went to a girls' school for awhile in highschool, and that was about my only REAL experience with choir except for a short stint in Germany. Since we had no men, I have a feeling some of the parts we were given were actually tenor rather than alto. In any case, I was surprised at how high the alto went in Messiah!
Yeah there are some parts which were difficult for me the last time I did a 'sing-in'. But I've noticed that my range has gotten higher over the last few years because of singing along with CDs I have and music we've done at Church. I'm not a 2nd soprano by any stretch of the imagination, but I can do the higher alto parts much easier than the lower ones these days. Again, it's more a function of the volume or projection rather than being able to reach the notes. I noticed I can reach all but the highest notes on "In Dreams" on the FoTR soundtrack and I would not have been able to do a few years ago.