As we know, in the past Blizzard has filed for US trademarks on the names of its upcoming Warcraft expansions (and other games) in advance of a major announcement (e.g., at Blizzcon). This has led to the expansion name getting out into the public domain earlier than Blizzard probably wants - and starting the rumor mill.
It seemed that this pattern was not repeated for Hearthstone, which was announced at PAX East on March 22, 2013. Blizzard did not file for a US trademark on the "Hearthstone" name at that time.
However, a check of the US Trademark Office database reveals that Blizzard filed a US trademark application for Hearthstone on July 7, 2013. Why wait so long? Wouldn't that encourage others to file on or use the same name before Blizzard (and potentially gain priority)?
Well, it seems that the US application for "Hearthstone" claims priority to February 28, 2013 (thereby predating the PAX announcement) via a related trademark filing in Swaziland! See: http://tsdr.uspto.gov/#caseNumber=86...e=statusSearch
Under 44(d), Blizzard has up to 6 months from the earlier foreign filing to file in the US and retain the earlier filing date as a US priority date. There does not seem to be any apparent reason to file in Swaziland first unless Blizzard intentionally wanted to delay filing in the US, thereby "hiding" the name from public release in the US Trademark database until it could make the press announcement at PAX.
What does this mean for the next WoW expansion? Well, because we're less than 4 months out from Blizzcon 2013, it's distinctly possible that Blizzard has already filed for trademark protection on the next WoW expansion name in some out-of-the-way jurisdiction like Swaziland and will wait until after Blizzcon to file in the US so the name isn't out there until Blizzard wants it to be.
At this point, I can't seem to find any publicly available trademark databases in Swaziland to see if other Blizzard filings exist.