I agree that the current PDO implementation lacks a portable way to quote SQL identifiers like table or field names. 
Some people will argue that in most cases it's better to avoid quote identifiers at all, and I agree. But every database has it's own list of reserved words which can't be used as a field or table name unless quoted. As it is hardly possible to avoid all reserved words from all databases (to improve portability between database backends), implementing such a method in PDO would be a big help.

Alternative suggestion: instead of adding a new method quoteIdentifier() extend PDO::quote() method to accept a new const PDO::PARAM_IDENTIFIER which works as follows:

$sql = sprintf("SELECT %s FROM %s",
    $pdo->quote('field', PDO::PARAM_IDENTIFIER),
    $pdo->quote('table', PDO::PARAM_IDENTIFIER)
);

$sql would then be 
for MySQL backend: 
SELECT `field` FROM `table`

for SQLite:
SELECT 'field' FROM 'table'

This would be especially handy if you try to extend PDO to allow extended placeholder syntax. In that case you normally simply imply that ` is used as field quote and thus defeat the purpose of using PDO - it isn't cross-DB-compatible anymore.

Hopefully this important (IMHO) feature will be added before we end up in a geriatric ward.
It has only been 8 years so far.