How common is the expression 'to keep someone across' the news. Is this a new phrasal verb? I've noticed it mostly in the last four years on British news programmes, such as the BBC. It seems to mean that they will try to keep us informed of any developments in a news story. Has it appeared in any dictionary yet?
You heard "keep someone across" the news correctly. It is not so common (evidenced in part by the response you've received here) and you are not the first to wonder about it (as you can see in this wordsmith.org forum as well as this wordreference.com forum)
However, yes, it is currently being used; here are a couple examples (with links):
"We keep you across events unfolding after yesterday's plane crash in eastern Ukraine." - from BBC World Service
There are several others you can find - all from UK, Australia, or NZ and all relatively recent - simply by Googling "keep you across" (with quotation marks) and hitting "news" (as oppposed to "web"). Interestingly, there are only three pages of results, which would suggest that the history of this expression (or, to be perfectly logical, the history of the use of the expression with "you"), is relatively brief.
The meaning in the examples you can find is, in almost all cases, "keep you abreast of" something, as defined in FumbleFingers post.
However, you will find it used in a slightly different sense here, in a review of LG's G Watch R, which is touted as a...
But no, you won't find it in dictionaries and it would seem, given the evidence, that we are witnessing the birth of a new expression. If anyone has evidence to show that this is, in fact a revival of an older expression, I'd love to see it!
I suspect OP has simply misheard something like The BBC keeps you abreast of current affairs...
abreast 2: up to a particular standard or level especially of knowledge of recent developments
keeps abreast of the news
...or maybe one or more other people have made this mistake, which OP has noticed. It doesn't seem like a very justifiable "spatial metaphor" usage to me in this context. Noting OP's example usages in a comment, I would expect...
"We'll keep you abreast of that story"
"We'll keep on that for you"
As John Lawler points out, unless anyone is prepared to accept forms like "We are keeping across Ken Anderson on this story", it wouldn't actualy be a "phrasal verb" anyway, because it doesn't do Particle Shift.