Starcraft II – Are you stuck in a league once you are placed

At the time of this writing there are:

Type      Total     Russia  Taiwan   SE Asia   Europe  Korea    NA    Latin America 
 All      25,670     862     1,707    1,299     7,371  2,678  11,380       373

Broken down by Type and League (approximate numbers)

 X          1v1         2v2         3v3         4v4        2Rand        3Rand        4Rand
Diamond     700         200          50          25         200          200          100
Plat       1400         700         200          50         400          400          300
Gold       1900        1100         400         200         700          700          500
Silver     1900        1300         500         200         800          700          500
Bronze     3500        2600        1600         600        1400          800          400
Total      9400        5900        2700        1050        3500         2800         1800

This data was tabulated by myself based on the website: http://sc2ranks.com/

As to what that split represents, I refer you to this question: What percentage of players get into each league tier

The short answer is: Points are not Comparable across leagues.

The accumulation of points is supposed to be proportional to the difference in skill between players. Unfortunately, the points you gain are unrelated to your ELO style rank. Because of things like: Bonus points, starting off at 0 points, and league promotion devaluing points, points don't correspond to a true ranking.

Instead, sites like SC2Rankings and Starcraft.org realize that the hidden ELO style rank that Blizzard uses is only accurately reflected in promotion between leagues. As a result, the best indicator of skill is what league a player is in. Since points are comparable inside of a league, the secondary sort these sites often use is that point metric (despite its deep flaws).

I'll probably write a longer explanation of how an ELO style ranking system should work to highlight why points don't, but the bottom line is still going to be: points aren't comparable between leagues.