As I read Planet13's comment, his thought wasn't to substitute synonyms to for the main keyword, which I agree would be a mistake, but rather to include them as extra vocabulary.
Exactly what I MEANT to say, Robert. Thank you for clarifying.
Just in the very limited studies I have done over the last few months, I have seen where a synonym in the title FOLLOWING the main keywords has done quite well - and I have seen some pages rank for two- and three-keyword phrases where the synonym appeared in lieu of one of the main keyword. (An example of this was where the three-word keyword phrase had the word dress in it but the #1 result had the word gown in lieu of dress in the title).
What I was looking at were product pages where the content was more or less the same because several retailer web sites were using the description provided by the manufacturer - hence many sites with extremely similar content.
Use of synonyms also raises questions about co-occurrence effects of additional vocabulary beyond your core phrases in a title.
Again, exactly what I WOULD HAVE said, if I were more articulate.
But one other thing, besides the importance of co-occurence effects, is that sometimes people just use different vocabulary.
In the States, one famous example is that lots of people out West say "soda" when referring to a carbonated soft drink, while in the North East, it is common to use the word "pop" where referring to a carbonated soft drink.
So using both words in the title just seems like you are casting a bigger net to catch more fish. And hopefully, because of the co-occurence that Robert mentioned, it wouldn't dilute the influence of the main keyword.
Maybe my idea is too simplistic. I realize that lots of SEO folk spend a lot of time finding that ONE KILLER KEYWORD that is searched for often, has little competition, and has lots of sites where they can get links to it.