Is it possible for others to use my username on message boards and reviews?

The book answer is two people can't have the same handle or use the same alias.  That is, of course, assuming your password or computer isn't compromised.

BUT there's a trick to it,and it applies only to certain aliases. (I won't elaborate further).

Certain Usernames too can be mimicked, BUT the one doing the forgery will NOT have a "little envelope" by their usernames for PM ing.

reynolds, if they forge your username, they are probably using the letter capital O instead of the number zero, also watch for the cap i instead of small L. While not exact, they look alike.