Now that MySQL 5.1 is the stable production version and 5.2 is the .net connector, this error occurs with the current (2/23/08) download of mojoportal. 

2.2.7.8 - 00:00:00.3750000 - MySql.Data.MySqlClient.MySqlException: Can't create table 'xxxx.mp_sites' (errno: 150)

So, how does one get around this, since I'm using the recommended production versions of each?  Please don't tell me to go backward.  Are there manual scripts that I can run to get the tables established?  Is there a change to source that is needed and recompiled to make it work?

Please point me in the right direction and I'll share my experience when done.

Steps to take:

1. Check the version of the mysql script from Setup\applications\mojoportal-core\SchemaInstallScripts\mysql\ -- navigate to
\Setup\applications\mojoportal-core\SchemaUpgradeScripts\mysql  and delete all scripts upto and including the same version found in the schemainstallscripts subdirectory -- in my case, that meant all scripts up to and including 2.2.7.8.config.
2. Copied the mysql script from Setup\applications\mojoportal-core\SchemaInstallScripts\mysql\2.2.7.8.config into the MYSQL Query Browser.  Had to execute on each line sequentially.  All query statements executed.
3. Followed Joe's advice, deleted update .config files through 2.2.7.8.config from the subdirectory \Setup\applications\mojoportal-core\SchemaUpgradeScripts\mysql -- re-entered http://xxxx/Setup/Default.aspx into browser.
Reached this statement:
Setup is complete, click the link below to go to the home page.
Database PlatformMySQL
Version2.2.8.6
StatusYour installation is up to date.
4. clicking on link opened up portal at http://xxxx/default.aspx

Done the above works. 

Error messages you might encounter if you don't delete the older script files:
File system permissions ok.
MySQL database connection ok.
database permissions are sufficient to alter schema.
database initial schema already exists.
database core schema needs upgrade.
0 site(s) found.
Running script mojoportal-core - 2.2.1.5 - 00:00:00.7812500
Running script mojoportal-core - 2.2.1.6 - 00:00:00.8750000
Running script mojoportal-core - 2.2.1.7 - 00:00:00.9687500
Running script mojoportal-core - 2.2.1.8 - 00:00:01.0781250
Running script mojoportal-core - 2.2.1.9 - 00:00:01.1718750
MySql.Data.MySqlClient.MySqlException: Duplicate column name 'FailedPwdAnswerAttemptCount' at MySql.Data.MySqlClient.MySqlStream.OpenPacket()

If you get errors similar to these, then go back and delete the scripts as instructed in step 1.

The following issues are un-resolved:

PS -- It is curious in this approach that the system is not aware of the version so that it can appropriately run the upgrades.  So, to avoid the mess I began with, deleting the scripts from the schemaupgradescripts directory up to the same version number that was in setup enabled the install script found using setup/default.aspx to operate as expected. My belief is that this deletion process shouldn't be required, since the script should be aware of what version it ran to begin with and would incrementally begin at the next number in sequence.

PPS - I did not have to do a manual insert of application ID information, since this process used the scripts as intended by mojoportal -- the scripts setup this table and the appropriate column information.

PPPS - still not sure why MySQL5.1 didn't work with the initial setup -- if I figure out more, I'll let this conversation know through a separate reply.

And, yes, I just update this post until it gets to the point that it has coherence -- that way this isn't a long list of disjointed information./
And no, I want to use the latest versions of MySQL -- I have other dbs that work just fine and use some of the newer features, so don't want to go backward.
And yes, I am listening, but it isn't a real-time connection that I have with this site or my email.  So there is a delay in reply.  Please be patient. 
Once done, I'll clean up this post so that it will make sense to anyone else following the same footsteps.

Post is now cleaned up.

Thanks for the explanation of the script issue.  I suspect had I followed the advice in my post, the script issue wouldn't have surfaced.  All is well.