The decision of the only currently available cross realm is only for people whom have purchased the South East Asia version. The decision by Blizzard is due to latency indifference generated between the two servers. Below is the FAQ from the Starcraft SEA section at Facebook.

Q: Who is eligible to play on the North American servers?
A: All players, including those in the regions of Hong Kong and Macau, who purchase the English Southeast Asia/Australia/New Zealand Standard or Collector’s Edition of StarCraft® II: Wings of Liberty™ will have the option of playing on the North American Battle.net® servers in addition to the Southeast Asia/Australia/New Zealand servers if they prefer.

Q: How much will this cost?
A: The option of playing on North American Battle.net servers will come at no additional cost to all players who have purchased the Southeast Asia/Australia/New Zealand version of StarCraft II.

Q: How will access to the North American servers be granted?
A: Our intention is to ensure that within 60 days after the launch of StarCraft II: Wings of Liberty, all players with a copy of the Southeast Asia/Australia/New Zealand version will automatically gain the ability to also access the North American servers -- this will be handled through Battle.net account management.

Q: How will I know when I can access the North American servers?
A: We will have additional information on how and when to access the North American servers on our StarCraft II community site.

Q: What will happen with my Southeast Asia/Australia/New Zealand Battle.net account?
A: Once you get access to the North American Battle.net servers, you will have access to both regions for online play. You can choose which one to play on at any given time, and can switch back and forth at will.

Q: How will my friends list and rankings be handled?
A: All account-specific data, including friends lists, rankings, achievements, statistics, titles, and unlockable rewards, will be tracked separately for each region's Battle.net servers, and will not carry over between them.

Q: If I have already purchased the North American version, can I also play on the Southeast Asia/Australia/New Zealand servers?
A: No. To have the option to play on both servers, you must purchase the Southeast Asia/Australia/New Zealand version of StarCraft II: Wings of Liberty. This solution is specifically targeted to address gamers in Southeast Asia/Australia/New Zealand who have social relationships with North American players from previous Blizzard Entertainment titles.

Q: Will I be able to play on any of the other StarCraft II: Wings of Liberty regional servers?
A: No. Players who purchase the Southeast Asia/Australia/New Zealand version of StarCraft II will have access to the Southeast Asia/Australia/New Zealand and the North American Battle.net servers. No other multi-region play will be available at this time.

However from this action, a lot of arguments are also actively rising up about cross realm play bias. Hopefully we will get full cross realm in the future patches or the next expansion in Heart of the Swarm.