The state of play is as follows:
First as noted above, you can only play in the region "on the box", ie. if you buy a Korean version of SC2, you can only play on the Korean region of bnet, even if you live in Lhasa. If you want to play with friends around the world, you will need to purchase multiple copies of SC2, one for each region in which you wish to play. (I know, I know.)
Second, Blizzard has explcitily stated that people with the SEA region version of SC2 will get access to the NA region. Using the same SEA client/copy of SC2, they will be able to connect to NA bnet and set up a NA bnet "character" (ie. achievements, play records etc will not be carried over, you will in effect have two entirely separate identities). This was promised within 60 days of release, most people seem to expect it to hit with a client update sometime in September (possibly although not necessarily at the same time as the 1.1 patch).
Third, Blizzard has also stated that they are aware of the desire for "cross-region" play, and that they wanted to have it available on release but technical difficulties ("ping" etc) made that impossible. They have said they would like to bring it to a future SC2 update. They have given no timetable for x-region play, nor (in contrast to two, above) have they definitively stated that x-region play will be available. They have just said "we know you want it" and "we would like to give it to you". For what it's worth I personally expect it will come eventually but "eventually" could be sooner or it could end up being much much later.
If you are travelling between regions, and want to be able to play "local", you just have to buy another copy.