Upgrading Dovecot v1.0 to v1.1

You can use your old dovecot.conf from v1.0. It should work without changes, although a couple of deprecated settings have been removed.

Connections

listen = [::] listens only for IPv6 connections now in most operating systems. If you want both IPv4 and IPv6 use listen = *, [::]
By default the number of connections for user from the same IP is limited to 10. You can change this from mail_max_userip_connections setting.

passwd-file: If you use %d in args, it no longer means that domain isn't looked up from the passwd-file. You'll need to add username_format=%n prefix to args (e.g. args = username_format=%n /etc/virtual.%d).
Empty or NULL password no longer means "any password is valid". You'll also have to return "nopassword" field.
PAM: There's no more blocking=yes setting, it's now always enabled. If you want to limit the number of lookups done by a dovecot-auth worker, change auth_worker_max_request_count setting. Setting it to 1 makes it work basically the same as the old blocking=no.
passwd: The problem with passwd lookups is that temporary errors (e.g. LDAP server down) are returned as "user doesn't exist" errors. You may want to try the new NSS userdb.
SQL and LDAP: user_global_uid and user_global_gid fields have been removed from their config files. Instead you can now use mail_uid and mail_gid settings in dovecot.conf. This also means that it's no longer a requirement to specify a userdb at all (a dummy static userdb is used internally).

In v1.0 mmap_disable=yes might have worked faster. If you had changed this only because of that, it's time to set it back to "no".
NFS users should now set mail_nfs_storage=yes and mail_nfs_index=yes. Dovecot no longer requires attribute cache to be disabled.
Quota plugin has completely new configuration. See Quota/New.
Maildir: dovecot-uidlist file is in a new format. The old format is automatically converted to new one, but if you plan to move back to v1.0 be sure to use at least v1.0.2 which will also understand this new format.
Index files have slightly changed as well. Upgrading to v1.1 should go transparently, but moving back to v1.0 might again cause some errors. v1.0.8 fixes some assert-crashes that were caused by reading v1.1-generated index files.
dotlock_use_excl=yes is default nowadays. If you're still using an ancient NFSv2 setup, you'll need to set this to "no".
mbox: Delete existing dovecot.index.cache files from all mailboxes. Otherwise you may see some errors in logs.