You're right, drs. Someplace is used rarely outside North America, and so should not be classed as a common word.
As a term meaning more or less the same as somewhere, it is sometimes written as two words, and sometimes as one. It seems it was always two words originally, but the one-word form has come to predominate. The phrase some place can always be used to mean "some location", in which case it is standard English everywhere. For example, "We might say in a modern home there should be some place where you can sit down and practice calming yourself..."
Someplace as a single word seems to have originated late in the 19th century. The Corpus of Historical American English has no examples before 1900, with usage building gradually during the 20th century, to peak in the 1970s. Its frequency in American usage seems to have been tapering off a little in recent years.
English usage guides published in the US seem to be divided in their opinion about whether someplace is appropriate in formal writing. Merriam Webster's Dictionary of English Usage considers it has "been common in general and even academic writing since at leat the 1940s".
Someplace appears about 3000 times in the Corpus of Contemporary American English, compared to somewhere with about 27,000 occurrences. So it is certainly not a rare word in the US. Out of interest, I checked in another corpus that has recently become available, the Corpus of American Soap Operas. Someplace seems to be more popular in this genre: the someplace / somewhere ratio is about 1 to 4.
However, in the British National Corpus, someplace appears only 18 times, and many of these are in dialogue in fiction, where the speaker may be meant to be American.
I found a few examples in the index of the Australian Fairfax newspapers, but quite a lot of them were in technology articles, where the writers might lean toward American idioms. One example from an experienced Australian journalist was in a 2010 article by Jo Chandler about the politician Bob Katter: "Finally he's reminded that he needs to be someplace else."
I will switch both someplace and anyplace to rare words. Noplace and everyplace are already labeled as rare.
[The backlog of unanswered word comments continues to grow. Fortunately, the absence of replies doesn't seem to deter people from sending in questions. It may seem unfair that I am answering the latest one while other forumites have been waiting patiently for weeks or months, but it's better that I answer something rather than be daunted by the task of finding the oldest suggestion.]
