There used to be a problem where people with dots early in their names were unnotifiable (because the dots would be stripped out and the name match would always fail), but it was fixed, so now dots matter. According to rule 3 of How do comment @replies work?:
If there are more than three characters in @name, then all given characters must match (neither @alix nor @aliceinwonderland will match user Alice).
Thus, at the time the question was asked,
@Pekka would match, but
@Pekka... would not. There was an exception to the rule where a single trailing punctuation character won't break the match, so
@Pekka, would work, but since there were three in this case it didn't apply at the time this question was asked.
In July 2014 this exception has been extended, and
@Pekka... now works as well.