So if it is classed as a planet . . .

. . . then what are the implications for astrology? After all, Pluto was fairly quickly assigned a significance partly based on the mythological nature of Pluto, thus demanding the question be asked "What if it had been named "Goofy" instead?
Since Sedna is named after the Inuit goddess of the ocean, it is fair to assume that when Sedna is prominent in a horoscope icebergs and polar bears may be a hazard.

As indeed the horoscope of the Captain of the Titanic, who had a large Sedna rising proves.
I don't believe in astrology -- but if I were treating astrology as a predictive model then I would say that an extra planet is great news. Astrologers use the presence or absence of the planets in houses to predict various things about your life. So if we assume this new thing is going to be accorded planet status (which is pretty bloody arbitrary anyway) then it would increase the predictive power of the model since it's another parameter.
No it wouldn't necessarily imply that at all... but some astrologers did make that claim you are right.
You could probably make a case for there being eleven "true" planets (and therefore another one for us to find) on thegrounds that there ought to be twelve astrological entities to match the twelve houses - ten other planets plus the Sun and the Moon. You can treat that as a prediction if you like (albeit an unfalsifiable one).
*baffle* Nine current planets plus supposed new planet + moon + sun... that would be ten true planets to make twelve. Or have I misunderstood you.

Of course there was another "new planet" discovered last year but people lost interest coz it was a bit titchy and crap.
Well, Earth probably doesn't count because it doesn't apparently move about, so you can't make predictions (or post-hoc rationalisations or whatever) based on that.
Well, Earth probably doesn't count because it doesn't apparently move about

You must drink less than me - I've definitely spotted the earth revolving around me.
Some astrologers predicted that there was a tenth planet... which gives them a bit of a "wahey" moment everytime some lump of ice and rock is discovered as a tenth planet. I imagine they were doing the same for the ninth planet too.

I'm narked that neither of my existing mnemonics for remembering the ordering of the planets can easily be extended to encompass Sedna:

(being "Mother Very Thoughtfully Made A Jam Sandwich Under No Protest" and "Space Men Vote Earth Most Jolly of the Solar Universe's Nine Planets")

Mother Very Easily Made a Jam Sandwich Using No Peanuts Mayonaise or Glue.

This enables us to remember:
Mercury, Venus, Earth, Mars, asteroid belt, Jupiter, Saturn, Uranus, Neptune, Pluto, Mickey (undiscovered as yet), Oort Cloud, Goofy (undiscovered as yet).
You have read Schrodinger's Cat and I claim my five pounds.

Or you and RAW have read the same source materials, of course.
I heard a brief quote on Today this morning to the effect that Mystic Meg was claiming that the hitherto-unkonwn new planet was the reason why astrologers "sometimes get it wrong". "Ha!" I thought.

But ISTR reading not so long ago that some astronomers thought that Pluto was barely big enough to be classified as a planet, and should be downgraded; and isn't Senga or whatever it's called smaller?
isn't Senga or whatever it's called smaller?

You do realise that's just crushed any chance I had of ever memorising it's real name?

You're not the first person to point this out. Back in 1977, much ado was made of the discovery of a "planet" between Saturn and Uranus. The body was later named Chiron, and it turned out to be a particularly large comet, but back in those halcyon days, astrologers were falling all over themselves to claim that they knew Chiron's influences. Naturally, all of that ado disappeared with time, and only a few astrologers even know about Chiron, much less include it in their charts.

And that brings up the question I asked myself back then: if astrology is allegedly a science, wouldn't astrologers have been able to ascertain Chiron's presence before its discovery by Charles Kowal? I did a bit more research, and discovered that in all three of the planets found in recent times, the same pattern repeated. In not one case could any astrologer say "I knew about Uranus/Neptune/Pluto before it was discovered" and confirm the fact. Now, I'm asking this when I was nearly 11 years old, and I still find it very hard to believe that any number of (presumably) wiser heads didn't ask that question about astrology a long time ago. In fact, it makes a great test: ask astrologers to find new planets in the Oort cloud, and aim telescopes at the areas where they claim the planets should be. If you find a body in that area at that time, matching the description the astrologer gave, the astrologer wins a prize.