Just to confuse things further and stating my position that these terms have different meanings in different parts of the country (world). From Wikipedia:
Mississippi straddle
A Mississippi straddle is similar to a live straddle, but instead of being made by the player "under the gun", it can be made by any player, depending on house rules. House rules permitting Mississippi straddles are common in the southern United States. Like a live straddle, a Mississippi straddle must be at least the minimum raise. Action begins with the player to the left of the straddle. If, for example (in a game with $10–25 blinds), the button puts a live $50 on it, the first player to act would be the small blind, followed by the big blind, and so on. If action gets back to the straddle with no raise, the straddle has the option of raising. The player to the left of a Mississippi straddle may re-straddle by placing a blind bet raising the original straddle.[4]
Sleepers
A sleeper is a blind raise, made from a position other than the player "under the gun".[5] A Mississippi straddle is a sleeper raise given this definition, but Mississippi straddles can be disallowed or restricted while sleepers are allowed at any position. A sleeper bet is not given the option to raise if other players call, and the player is not buying last action; thus the sleeper bet simply establishes a higher minimum to call for the table during the opening round and allows the player to ignore his turn as long as no-one re-raises the sleeper bet.
Sleepers are often considered illegal out-of-turn play and are commonly disallowed, but they can speed up a game slightly as a player who posts a sleeper can focus his attention on other matters such as ordering a drink or buying a tray of chips. It can also be an intimidation tactic as a sleeper raise makes it unfeasible to "limp in" (a situation where a player with a mediocre starting hand but acting late only has to call the minimum to see more cards), thus forcing weaker but improvable starting hands out of the play.
[edit] Examples
A game of no-limit poker with blinds of $1/$2. Alice is in the small blind, Bob is in the big blind, Carol is next to act, followed by David, with Ellen on the button.
Straddle: Alice posts $1, Bob posts $2, Carol posts a straddle of $4. The hole cards are dealt. Because of the straddle, David is now first to act; he folds. Ellen calls the straddle. Alice folds. Bob, the big blind, calls the straddle by putting an additional $2 in the pot. Carol has the option of checking or raising; she makes a raise of $8. Ellen folds. Bob calls the raise, ending betting on this round.
Mississippi straddle: Alice posts $1, Bob posts $2, Ellen, on the button, posts a Mississippi straddle of $4. Because of the straddle, Alice, the small blind, is now first to act; she folds. Bob calls the straddle by putting an additional $2 in the pot. Carol folds. David calls the straddle. Ellen has the option of checking or raising; she checks, ending betting on this round.
Sleeper: Alice posts $1, Bob posts $2, and David posts a sleeper blind of $4. The hole cards are dealt. Carol acts first as last action remains with the big blind, but the bet to her is $4. She calls. There is no additional bet to David and he has no option, so play passes over him to Ellen. She calls the $4 as well. Alice folds. Bob, in the big blind, no longer has the option either; he must either call $2, raise, or fold. He raises by $4 (total bet is now $8). Carol re-raises to $12. The bet is now $8 to David, who must now call, raise or fold; he calls, as do Ellen and Bob, ending the betting round.
