The first interesting thing about this statement is the bizarre phrasing of its opening: والصلاة والسلام على الضحوك القتال سيدنا محمد صلى الله عليه وسلم. The Guardian renders this as "may peace be upon the cheerful one and undaunted fighter, Prophet Muhammad, God's peace be upon him." The doubling of "peace be upon him" (a formula added to the prophet's name as a matter of course) is unusual [because of its redundancy] and stylistically flawed, suggesting an imperfect command of Arabic literary style. The phrase الضحوك القتال (ad-Ḍaḥûk al-Qattâl), rendered by the Guardian as "the cheerful one and undaunted fighter", is composed of two words in apposition which Hans Wehr's dictionary renders as "frequently, or constantly, laughing; laugher" and "murderous, deadly, lethal". This extremely unusual epithet is so weird that at first sight I assumed it must be some kind of prank; it may potentially provide some clues to the identity of the killers.
Such an opening has been used at least once before in Europe: the assassin of Theo van Gogh left a note on the body opening after the standard invocation of God's name, with Vrede en zegeningen op de Emir van de Mujahideen, de lachende doder Mohammed Rasoeloe Allah (Sala Allaho alaihie wa Sallam), ie "Peace and Blessings
The third really weird thing about the message is the phrase ابشرى با أمة الاسلام ابشرى يا امة العروبة : "Rejoice oh community of Islam, rejoice oh community of Arabdom". This collocation itself appears to be well-established, if rare - the phrase "community of Arabdom" (ummat al-`Urûbah) gets only 37 google hits, but many are collocations of one sort or another with "community of Islam", and come from speeches or interviews by well-known politicians. However, it does not seem to form any part of the standard rhetoric of so-called "jihadists".
PS: Juan Cole explores, among other things, the implications of the "Arabdom" phrase.
PPS: Shibli Zaman also examines the linguistics of the issue; his summary of the "urubah" issue is more detailed than mine.