So I'm testing Drupal 7 alpha and coming across with some interesting issues so far on upgrading. Keeping in mind that this is an alpha release, I have no expectations whatsoever that anything will work on upgrades. That said, here are my issues:
Views and update.php
When updating the database scripts using update.php, I was getting a blank page. Following some recommendations from users having issues on Drupal 6, I edited update.php and commented out line 326, ini_set('display_errors', FALSE);. Commenting this out displays all errors when running the script, which in this instance told me there was an issue with Views. If I disable views (uncheck all of the sub-modules as well) the script runs fine. Weird. I believe this has something to do with the .inc file Views is trying to pull up, but I'm no PHP ninja.
When going from alpha2 to alpha3, a host of modules went from enabled to disabled. This didn't affect the content at all (to my knowledge), but I can't find a reasonable explanation for this. Blog, CCK, PathAuto, and a few other modules disabled themselves on the upgrade.