When counting the entropy you would probably count each word as a single entry, and each special character as an entry (and disregard spaces).
* By capitalizing the words you've doubled the search space for words (assuming that the search space starts with all words lowercased)
* You could increase the search space for each word by 200% (from the space of all lowercase words) by including the possibility of words in all caps (it's unlikely for people to start using alternating case in the middle of words).
* The ' in "Don't" doesn't increase the search space that much because there are a small number of (common) contractions like that, and each of them would only break down into 3 permutations:
don't
dont
don t
(though the last one is highly unlikely). So you're adding maybe 30 more words to a search space much large than that.
* As far as the special character is concerned, it probably doesn't add too much to the search space. You can break down your phase like so:
Don't Forget About Poland!
||
\/
{item} {item} {item} {item}{item}
||
|| Disregard whitespace (acquire entropy!)
\/
{item}{item}{item}{item}{item}
So now you've got 5 items. Each item could be either a word or punctuation. The search space for words is
huge. The search space for punctuation is small. Your algorithm just has to realize that if it chooses punctuation for one of the items, then it doesn't bother to use whitespace to separate it from the preceding word ("word," vs "word ,").
* You can also further reduce the effects of punctuation on the search space by realizing that punctuation will almost always follow a word, and not other punctuation. This also discounts punctuation as the first item in the passphrase too.
Edit:
Upon further though, if the attacker uses a simplified algorithm to account to upper-/lowercase, then it may not have that much of an effect on the search of each individual item (i.e. n!4 instead of (n+4)!). An attacker could break the common instances of case down into:
* All words lowercased "don't forget about poland!"
* All words uppercased "DON'T FORGET ABOUT POLAND!"
* All words titlecased "Don't Forget About Poland!"
* First word titlecased "Don't forget about poland!"
This discounts the possibility of people alternating titlecase across words, because that's probably as likely to happen as people alternating case within words (e.g. WoRdS lIkE ThIs). Granted, this also discounts proper nouns in the middle of the passphrase (things that don't require extra effort for people to remember to capitalize).