Regarding Bulgaria, I started a thread a while ago -
http://www.europa-universalis.com/forum/showthread.php?s=&threadid=74493
it has some good links in my opinion on the subject of bulgarian participation in WW2 (unfortunatelly there are not so many). Burris is absolutely right, when he says that Bulgaria never declared war to USSR, nor did Bulgaria ever sent any forces to Russia. I've studied Bulgarian participation in the war for years (I'm a student in International Relations in Sofia) and I've never seen any postive info of a single bulgarian soldier sent to Russia. There were of course some clashes in 1941 in the Black sea region, where Soviet subs and planes attacked Bulgarian merchant shipping, Bulgarian planes attacking Soviet subs there, Soviet subs unloading Bulgarian communist groups on bulgarian shore, and some unconfirmed reports about Soviet planes bombing Dobrich in June 1941. But the fact is that there was a Bulgarian embassy in Moscow in 1941-1944, and the countries were not officially at war until 6th of September, 1944, when Soviet Union declared war on Bulgaria, and even then the war "lasted" only several days, as on 9th the Communist uprising in Sofia took place, and the communist came to power. The interesting point is that on 5th of September, the new Bulgarian government had already declared a war on Germany, thus for a brief period Bulgaria was at war with US, UK, USSR and Germany.
Also Burris is right that Czar Boris III death increased seriously the level of dissent in Bulgaria. The King was a popular figure, but his authoritarian style of ruling Bulgaria resulted in sidelining and alienating all major political leaders, and after his death, the system of government he established, began to disintegrate slowly. The anti-fascist resistance, insignificant till late 1943, began to gather strength. The fact that by 1943 it was clear that the Germany was again losing the war, and Bulgaria was again on the losing side; and the start of the allied bombing raids over Bulgaria, also added fuel to the dissent.
About whether Boris was poisoned and by whom, there are different theories. Some claim it was the Germans, some blaim the British intelligence, or the Communists, some say his death was of natural cause.