Warbler Guy, has the Arctic Warbler been seen in California? Has the Arctic Warbler split into three species?

Yes and yes, Alex (in Cincinnati).

Based on blood analysis, and per recent passage by the American Ornithologists' Union Committee on Classification and Nomenclature, the Arctic Warbler complex consists of the Alaskan breeding Arctic Warbler (that ranges across northern Eurasia as a nester)  Phylloscopus borealis  (including the previously named subspecies forms P. b. kennicotti, talovka, transbaicalicus and hylebata). 

Two newly-defined species — the Kamchatka Leaf Warbler Phylloscopus examinandus and the Japanese Leaf Warbler Phylloscopus xanthodryas — both split from the Arctic Warbler P. borealis.

The Kamchatka Leaf Warbler breeds in southern Kamchatka, Sakhalin, Hokkaido and the Kurile Islands. The Japanese Leaf Warbler breeds in Japan (except Hokaido). 

Although all three species — the Arctic Warbler, the Kamchatka Leaf Warbler, and the Japanese Leaf Warbler — look similar in appearance, differences in their songs and calls were determining factors in the latest reorganization. 

As for California sightings of the Arctic Warbler, eight accepted occurrences are listed at the California Birds Record Committee web site, including the most recent one on southeast Farallon Island in 2012. 

References:

1. http://ebonph.wordpress.com/2014/05/01/ask-the-experts-the-arctic-warbler-splits/

2. http://www.birdwatch.co.uk/categories/articleitem.asp?item=741

Warbler Guy, which birding web site is good to note rare warbler sightings? Rare bird alerts appear at one web site?

Ellie (in Boston), I think you may appreciate:

birdingonthe.net

It hosts ongoing Rare Bird Alert updates from around the country.

So, for example, if I wish to see a summary of rare birds called into the northern California hotline,
I go to this web site.

Here, the latest transcripts from 10/25 - 11/9/14 are featured.

No rare warbler sightings are present in this compendium, but it's still interesting to see reports of

CACKLING GOOSE, FALCATED DUCK, LEWIS'S WOODPECKER, 
TROPICAL KINGBIRD, CLAY-COLORED SPARROW (possible), WHITE-THROATED SPARROW, 
CHESTNUT-COLLARED LONGSPUR, and SNOW BUNTING.