I guess u have forget to add the method to support mb_string for strpos(), while u did it for substr() and strlen(). i have to say it is crucial, as without it the utf-8 post excerpt displays flawly, and some even generate infinite loops. and it seems some code contains strlen() remains where the new $this->strlen() should take place.
asides, does stripshortcode() really stripe [embed][/embed], which is the new feature of wp 2.9? All other short codes work fine (been stripped) except [embed], oddly.