As a follow up to the series of posts I've been making, I wanted to post what I ended up with.  Thanks to everyone who posted comments, your help was extremely useful. To recap, I have a working pair of events to add and remove partitions to this table:

create table log (
    logged datetime not null,
    id int not null auto_increment,
    text varchar(256),
    PRIMARY KEY ( logged, id )
)
PARTITION BY RANGE( TO_DAYS( logged ) ) (
    PARTITION p20080206 VALUES LESS THAN (733444),
    PARTITION p20080207 VALUES LESS THAN (733445),
    PARTITION p20080208 VALUES LESS THAN (733446)
);

The partitions are keyed off of the TO_DAYS function on the date/time of the log entries. Every day I want to create a new partition for tomorrow. It's important that we create this a day in advance so new entries at 12:00AM that day do not get a "partition not found" error. Our event to do so looks like this:

DELIMITER $
CREATE EVENT log_addpartition ON SCHEDULE EVERY 1 DAY
DO
  BEGIN
    DECLARE new_partition CHAR(32) DEFAULT
      CONCAT ('p', DATE_FORMAT(DATE_ADD(CURDATE(), INTERVAL 1 DAY), '%Y%m%d'));
    DECLARE max_day INTEGER DEFAULT TO_DAYS(NOW()) +2;

    SET @s = 
      CONCAT('ALTER TABLE log ADD PARTITION (PARTITION ', new_partition,
      ' VALUES LESS THAN (', max_day, '))');
    PREPARE stmt FROM @s;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;
  END $
DELIMITER ;

DELIMITER $
CREATE EVENT log_removepartition ON SCHEDULE EVERY 1 DAY
DO
  BEGIN
    DECLARE old_partitions CHAR(64) DEFAULT '';
    SELECT CONCAT( 'ALTER TABLE log DROP PARTITION ', 
        GROUP_CONCAT( PARTITION_NAME ))
    INTO @s
    FROM information_schema.PARTITIONS
    WHERE   TABLE_SCHEMA=schema() AND
            TABLE_NAME='log' AND
            CREATE_TIME < DATE_SUB( CURDATE(), INTERVAL 1 WEEK )
    GROUP BY TABLE_NAME;

    PREPARE stmt FROM @s;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;
  END $
DELIMITER ;

I can check the status of the events in the Information Schema:

mysql> select * from INFORMATION_SCHEMA.EVENTS\G
*************************** 1. row ***************************
       EVENT_CATALOG: NULL
        EVENT_SCHEMA: test
          EVENT_NAME: log_addpartition
             DEFINER: root@localhost
           TIME_ZONE: SYSTEM
          EVENT_BODY: SQL
    EVENT_DEFINITION: BEGIN
    DECLARE new_partition CHAR(32) DEFAULT
      CONCAT ('p', DATE_FORMAT(DATE_ADD(CURDATE(), INTERVAL 1 DAY), '%Y%m%d'));
    DECLARE max_day INTEGER DEFAULT TO_DAYS(NOW()) +1;


    SET @s =
      CONCAT('ALTER TABLE log ADD PARTITION (PARTITION ', new_partition,
      ' VALUES LESS THAN (', max_day, '))');
    PREPARE stmt FROM @s;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;
  END
          EVENT_TYPE: RECURRING
          EXECUTE_AT: NULL
      INTERVAL_VALUE: 1
      INTERVAL_FIELD: DAY
            SQL_MODE:
              STARTS: 2008-02-20 07:05:05
                ENDS: NULL
              STATUS: ENABLED
       ON_COMPLETION: NOT PRESERVE
             CREATED: 2008-02-20 07:05:05
        LAST_ALTERED: 2008-02-20 07:05:05
       LAST_EXECUTED: 2008-02-20 07:05:05
       EVENT_COMMENT:
          ORIGINATOR: 1118021742
CHARACTER_SET_CLIENT: latin1
COLLATION_CONNECTION: latin1_swedish_ci
  DATABASE_COLLATION: latin1_swedish_ci

*************************** 2. row ***************************
       EVENT_CATALOG: NULL
        EVENT_SCHEMA: test
          EVENT_NAME: log_removepartition
             DEFINER: root@localhost
           TIME_ZONE: SYSTEM
          EVENT_BODY: SQL
    EVENT_DEFINITION: BEGIN
    DECLARE old_partitions CHAR(64) DEFAULT '';

    SELECT CONCAT( 'ALTER TABLE log DROP PARTITION ',         GROUP_CONCAT( PARTITION_NAME ))
    INTO @s
    FROM information_schema.PARTITIONS
    WHERE   TABLE_SCHEMA=schema() AND
            TABLE_NAME='log' AND
            CREATE_TIME < DATE_SUB( CURDATE(), INTERVAL 1 WEEK )
    GROUP BY TABLE_NAME;

    PREPARE stmt FROM @s;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;
  END
          EVENT_TYPE: RECURRING
          EXECUTE_AT: NULL
      INTERVAL_VALUE: 1
      INTERVAL_FIELD: DAY
            SQL_MODE:
              STARTS: 2008-02-20 07:06:48
                ENDS: NULL
              STATUS: ENABLED
       ON_COMPLETION: NOT PRESERVE
             CREATED: 2008-02-20 07:06:48
        LAST_ALTERED: 2008-02-20 07:06:48
       LAST_EXECUTED: 2008-02-20 07:06:48
       EVENT_COMMENT:
          ORIGINATOR: 1118021742
CHARACTER_SET_CLIENT: latin1
COLLATION_CONNECTION: latin1_swedish_ci
  DATABASE_COLLATION: latin1_swedish_ci
2 rows in set (0.01 sec)

And finally, what does our table actually look like?

select DATE_FORMAT(logged, '%m/%d/%Y'), count(*) from log group by TO_DAYS(logged);
+---------------------------------+----------+
| DATE_FORMAT(logged, '%m/%d/%Y') | count(*) |
+---------------------------------+----------+
| 02/13/2008                      |     1103 | 
| 02/14/2008                      |     2158 | 
| 02/15/2008                      |     2878 | 
| 02/16/2008                      |     2880 | 
| 02/17/2008                      |     2880 | 
| 02/18/2008                      |     2878 | 
| 02/19/2008                      |     2880 | 
| 02/20/2008                      |      892 | 
+---------------------------------+----------+
8 rows in set (0.02 sec)

Note we have 8 days in this table because in our 'log_removepartition' event because we search for all CREATE_TIMEs that are greater than 1 week ago (not greater than and equal to).