diff --git a/django/db/backends/mysql/validation.py b/django/db/backends/mysql/validation.py
index 85354a8468..3014d7bae5 100644
--- a/django/db/backends/mysql/validation.py
+++ b/django/db/backends/mysql/validation.py
@@ -2,12 +2,27 @@ from django.db.backends import BaseDatabaseValidation
 
 class DatabaseValidation(BaseDatabaseValidation):
     def validate_field(self, errors, opts, f):
-        "Prior to MySQL 5.0.3, character fields could not exceed 255 characters"
+        """
+        There are some field length restrictions for MySQL:
+
+        - Prior to version 5.0.3, character fields could not exceed 255
+          characters in length.
+        - No character (varchar) fields can have a length exceeding 255
+          characters if they have a unique index on them.
+        """
         from django.db import models
         from django.db import connection
         db_version = connection.get_server_version()
-        if db_version < (5, 0, 3) and isinstance(f, (models.CharField, models.CommaSeparatedIntegerField, models.SlugField)) and f.max_length > 255:
-            errors.add(opts,
-                '"%s": %s cannot have a "max_length" greater than 255 when you are using a version of MySQL prior to 5.0.3 (you are using %s).' % 
-                (f.name, f.__class__.__name__, '.'.join([str(n) for n in db_version[:3]])))
-    
\ No newline at end of file
+        varchar_fields = (models.CharField, models.CommaSeparatedIntegerField,
+                models.SlugField)
+        if isinstance(f, varchar_fields) and f.max_length > 255:
+            if db_version < (5, 0, 3):
+                msg = '"%(name)s": %(cls)s cannot have a "max_length" greater than 255 when you are using a version of MySQL prior to 5.0.3 (you are using %(version)s).'
+            if f.unique == True:
+                msg = '"%(name)s": %(cls)s cannot have a "max_length" greater than 255 when using "unique=True".'
+            else:
+                msg = None
+
+            if msg:
+                errors.add(opts, msg % {'name': f.name, 'cls': f.__class__.__name__, 'version': '.'.join([str(n) for n in db_version[:3]])})
+
diff --git a/docs/ref/databases.txt b/docs/ref/databases.txt
index 79e6136104..0b5a819898 100644
--- a/docs/ref/databases.txt
+++ b/docs/ref/databases.txt
@@ -234,8 +234,11 @@ storage engine, you have a couple of options.
 
 .. _AlterModelOnSyncDB: http://code.djangoproject.com/wiki/AlterModelOnSyncDB
 
-Boolean fields in Django
--------------------------
+Notes on specific fields
+------------------------
+
+Boolean fields
+~~~~~~~~~~~~~~
 
 Since MySQL doesn't have a direct ``BOOLEAN`` column type, Django uses a
 ``TINYINT`` column with values of ``1`` and ``0`` to store values for the
@@ -244,6 +247,19 @@ of that field for more details, but usually this won't be something that will
 matter unless you're printing out the field values and are expecting to see
 ``True`` and ``False.``.
 
+Character fields
+~~~~~~~~~~~~~~~~
+
+Any fields that are stored with ``VARCHAR`` column types have their
+``max_length`` restricted to 255 characters if you are using ``unique=True``
+for the field. This affects :class:`~django.db.models.CharField`,
+:class:`~django.db.models.SlugField` and
+:class:`~django.db.models.CommaSeparatedIntegerField`.
+
+Furthermore, if you are using a version of MySQL prior to 5.0.3, all of those
+column types have a maximum length restriction of 255 characters, regardless
+of whether ``unique=True`` is specified or not.
+
 .. _sqlite-notes:
 
 SQLite notes 
diff --git a/docs/ref/models/fields.txt b/docs/ref/models/fields.txt
index 4f2a92a66b..52508c7e5c 100644
--- a/docs/ref/models/fields.txt
+++ b/docs/ref/models/fields.txt
@@ -327,6 +327,13 @@ The admin represents this as an ``<input type="text">`` (a single-line input).
     The maximum length (in characters) of the field. The max_length is enforced
     at the database level and in Django's validation.
 
+.. note::
+
+    If you are writing an application that must be portable to multiple
+    database backends, you should be aware that there are restrictions on
+    ``max_length`` for some backends. Refer to the :ref:`database backend
+    notes <ref-databases>` for details.
+
 .. admonition:: MySQL users
 
     If you are using this field with MySQLdb 1.2.2 and the ``utf8_bin``
@@ -341,7 +348,8 @@ The admin represents this as an ``<input type="text">`` (a single-line input).
 .. class:: CommaSeparatedIntegerField(max_length=None, [**options])
 
 A field of integers separated by commas. As in :class:`CharField`, the
-:attr:`~CharField.max_length` argument is required.
+:attr:`~CharField.max_length` argument is required and the note about database
+portability mentioned there should be heeded.
 
 ``DateField``
 -------------
@@ -654,9 +662,10 @@ Like a :class:`PositiveIntegerField`, but only allows values under a certain
 containing only letters, numbers, underscores or hyphens. They're generally used
 in URLs.
 
-Like a CharField, you can specify :attr:`~CharField.max_length`. If
-:attr:`~CharField.max_length` is not specified, Django will use a default length
-of 50.
+Like a CharField, you can specify :attr:`~CharField.max_length` (read the note
+about database portability and :attr:`~CharField.max_length` in that section,
+too). If :attr:`~CharField.max_length` is not specified, Django will use a
+default length of 50.
 
 Implies setting :attr:`Field.db_index` to ``True``.