To say that Jordan's relationship with the Palestinians is complex is a gross understatement. When the 1949 Armistice Agreements went into effect, Jordan occupied the largest swathe of territory that had been promised to Palestine in the 1947 Partition: the West Bank. By then, Jordan was already ruled by the Hashemites. For 19 years, Jordan controlled the West Bank and annexed the territory (an annexation recognized by only two countries: Great Britain and the United States). Since then, Jordan has been very ambivalent about integrating Palestinians into its society. Jordan's Hashemite government has been consistently worried about what will happen if the demographic balance tips too far in the Palestinians' favor and increases the likelihood of a coup. Over the past five years, Jordan has occasionally revoked citizenship to thousands of Palestinians, especially those that have recently migrated from the West Bank. Jordan, like Lebanon, has a very fragile demographic balance between its Palestinians, Bedouins and Hashemites. The government fears that any significant upset to this balance will likely create a civil war like Lebanon's. Indeed, this was part of the reason why Jordan renounced its claims to the West Bank in 1988 to avoid being saddled with an influx of Palestinian refugees if it was ever forced to exercise sovereignty over the West Bank.
So, what does Jordan want? On the one hand, the creation of a Palestinian state would allow Jordan to expel a large number of its Palestinians into the new Palestinian state. While many of Jordan's do not live in refugee camps, about 350,000 do. Even those who are not in camps are still given separate legal identification so they can be easily separated from non-Palestinian Jordanian citizens. The stated Jordanian goal of this "policy is to prevent Israel from emptying the Palestinian territories of their original inhabitants." Israel's Arab neighbors are "concerned" about Palestinians losing their identity by virtue of their being absorbed into mainstream Jordanian, Lebanese or Syrian society.
On the other hand, while countries like Jordan have much to fear from the emergence of a new Palestinian state primarily in the West Bank. For thing, many Palestinians living in Jordan may not want to return to the West Bank and there may be substantial conflict. This is likely to happen because (a) many Palestinians have significant roots in Jordan now, (b) many Jordanian Palestinians may be concerned about the ability of a newly formed Palestinian state to absorb them and (c) many Palestinians may not want to live under a more religiously strict government under Hamas.
To the extent that Jordan will want to empty itself of its Palestinians given the Hashemite animosity to Palestinians, the Jordanian government may also be concerned about the new Palestinian state's policy toward Jordan. For one, many senior PLO members remember the vengeance wreaked upon them by the Jordanian government in Black September. Second, the new Palestinian state will likely become an advocate for greater rights of Palestinians who remain located in Syria, Lebanon or Jordan and may be a source of tension, especially where the Jordan borders the West Bank and there may be attempts by Palestinians in Jordan to cross into the West Bank or vice versa.
The sudden emergence of a Palestinian state alongside Jordan, a country with a substantial Palestinian population, may put the Jordanian government in great fear for its continued survival. Right now, the PLO in the West Bank is largely disarmed, but to the extent it becomes militarized, it may look east to Jordan to try to expand its sphere of influence and liberate the 3,000,000 Palestinians in Jordan from the Hashemite yoke. King Abdullah has been taught by his late father about the dangers of Palestinian nationalism, and he may be just as concerned about those nationalist dreams coming true right across his border as he was with Black September.