Is it possible the column was populated and it is has since been cleared out, or the data type was recently changed to varchar(max)? Have you tried rebuilding the clustered index (which will touch every row, unlike some ALTERs)?

EDIT

Since rebuilding the clustered index didn't help, I'm at a loss, and since the size of the existing data is so small, I recommend just creating a new version of the table and moving the data over. You can do this by explicitly re-creating the table and then copying the data, or you can use SELECT INTO to minimize the steps.

SELECT ... other cols ...,
    varchar_max_col = CONVERT(VARCHAR(MAX), NULL) 
    INTO dbo.newtable FROM dbo.oldtable;
CREATE CLUSTERED INDEX ... ON dbo.newtable( ...cols... );
CREATE INDEX ... ON dbo.newtable( ...cols... );
...
GO
BEGIN TRANSACTION;
EXEC sp_rename 'dbo.oldtable', 'oldtable_backup', OBJECT;
EXEC sp_rename 'dbo.newtable', 'oldtable', OBJECT;
COMMIT TRANSACTION;

There is the Percona Tool called pt-online-schema-change

If you can tolerate downtime, you can try the following:

Suppose you have the following

• datadir is /var/lib/mysql
• MyISAM table named mydb.mytable
• You have large disk volume mounted on /backup

You could do the following

mysql -u... -p... -e"CREATE TABLE mydb.mynewtable LIKE mydb.mytable"
mysql -u... -p... -e"ALTER TABLE mydb.mynewtable ADD INDEX (acctnum)"
#
# Create Symlinks Pointing to Another Disk
#
cp /var/lib/mysql/mydb/mynewtable.MY[ID] /backup/.
rm -f /var/lib/mysql/mydb/mynewtable.MY[ID]
chown mysql:mysql /backup/mynewtable.*
ln -s /backup/mynewtable.MYD /var/lib/mysql/mydb/mynewtable.MYD
ln -s /backup/mynewtable.MYI /var/lib/mysql/mydb/mynewtable.MYI
mysql -u... -p... -e"INSERT INTO mydb.mynewtable SELECT * FROM mydb.mytable"
mysql -u... -p... -e"ALTER TABLE mydb.mytable RENAME mydb.myoldtable"
#
# Erase Symlinks
#
rm -f /var/lib/mysql/mydb/mynewtable.MY[ID]
#
# Move Newly Made Table Back
#
mv /backups/*.MY[ID] /var/lib/mysql/mydb/.
mysql -u... -p... -e"FLUSH TABLES"

Seems like a lot of work, eh ??? Try the pt-online-schema-change first.

UPDATE 2012-08-22 11:56 EDT

I am not sure what would happen, but please hear me out on this suggestion: Try executing a repair on an empty .MYI file.

How do you do that? Take my idea from my first suggestion and augment to swap the new and old .MYI.

mysql -u... -p... -e"CREATE TABLE mydb.mynewtable LIKE mydb.mytable"
mysql -u... -p... -e"ALTER TABLE mydb.mynewtable ADD INDEX (acctnum)"
cp /var/lib/mysql/mydb/mytable.MYI /backup/.
rm -f /var/lib/mysql/mydb/mytable.MYI
cp /var/lib/mysql/mydb/mynewtable.MYI /var/lib/mysql/mydb/mytable.MYI
mysql -u... -p... -e"REPAIR TABLE mydb.mytable"

This should perform an in-place linear index rebuild.

Give it a Try !!!