At first I thought that 'DBI:ODBC:xxxx' should be 'dbi:ODBC:xxx' but it appears after all these years of using DBI and writing DBDs I never knew "DBI" was valid.

I think upper case DBI is tolerated, but not necessarily valid. The DBI docs state:

The $data_source value must begin with "dbi:driver_name:". The driver_name specifies the driver that will be used to make the connection. (Letter case is significant.)

The wording is a little bit problematic here: Is letter case significant only for the driver name or for the entire data source?

At least once in parse_dsn, the DBI docs use a data source string starting with upper case "DBI":

($scheme, $driver, $attr_string, $attr_hash, $driver_dsn) = DBI->parse
+_dsn("DBI:MyDriver(RaiseError=>1):db=test;port=42");
$scheme      = 'dbi';
$driver      = 'MyDriver';
$attr_string = 'RaiseError=>1';
$attr_hash   = { 'RaiseError' => '1' };
$driver_dsn  = 'db=test;port=42';
[download]

Looking at the result of the example, at least parse_dsn() should ignore case for the scheme part of the data source.

It does so at least on Windows (DBI 1.607) and on Linux (DBI 1.6.16):

C:\>perl -MDBI -MData::Dumper -e "print qq[$DBI::VERSION\n],Dumper([DB
+I->parse_dsn('DBI:foo:bar')])"
1.607
$VAR1 = [
          'dbi',
          'foo',
          undef,
          undef,
          'bar'
        ];

C:\>perl -v

This is perl, v5.10.0 built for MSWin32-x86-multi-thread

Copyright 1987-2007, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using "man perl" or "perldoc perl".  If you have access to
+ the
Internet, point your browser at http://www.perl.org/, the Perl Home Pa
+ge.

C:\>
[download]

> perl -MDBI -MData::Dumper -e 'print "$DBI::VERSION\n",Dumper([DBI->p
+arse_dsn("DBI:foo:bar")])'
1.616
$VAR1 = [
          'dbi',
          'foo',
          undef,
          undef,
          'bar'
        ];

> perl -v

This is perl 5, version 12, subversion 3 (v5.12.3) built for x86_64-li
+nux-thread-multi

Copyright 1987-2010, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using "man perl" or "perldoc perl".  If you have access to
+ the
Internet, point your browser at http://www.perl.org/, the Perl Home Pa
+ge.

>
[download]

Alexander