Poland fought extremely well, and only really fell because "promised" assistance from the UK and France never materialized
I have heard this before, but IMHO it is not a very good representation of the situation. Far more important, I think, was the strategic siutation Poland found itself in, as a result of having been created out of hide of Imperial Germany, having captured soil from the Bolsheviks, getting involved in the Silesian uprisings in the 1920s, and participating in the dismembering of Czechoslovakia by taking Teschen Province when the Germans rolled in to CZ in 1938. Poland had bad relations with everybody around them.
To make things worse, their geography was not very conducive to defense, as I am sure most readers here are well aware so I will not belabour it. Although they had developed a number of fortified areas and fortresses (Mlawa, Hel, etc..) these really didn't make much difference to the unfavourable geographic situation.
I do think that the Allies played a role, and a very damaging one, before September 1939. This role was their constant diplomatic pressure on the Poles to deter them from doing anything that might provoke Hitler. Partly as a result of this pressure, the Poles delayed the start of mobilization until later in the summer. Although they were able to achieve alot (ie: they preserved their Air Force by dispersing it to satellite fields) they were not completely ready when the Germans arrived in Sept 1939. Once the invasion began, the Allies still prssured the Poles to seek some sort of accomodation with the Germans. I have read an account of a Polish bomber squadron that was in the midst of arming and fueling for a raid on Konigsberg (keeping in mind that some Polish cities were already burning under German bombs), when an officer rushed out to the flight line with an order to stop, as the Allies had demanded that Warsaw not take any "offensive" actions against German territory. Of course, the ineffective Allied air operations (leaflet drops), as well as the farcical French "advance" into Germany in response to the attack on Poland, achieved nothing of significance. That the Poles lasted as long as they did, and caused as many German casualties as they did, is to their everlasting credit. Honestly though, I doubt that they could have defeated the Germans on their own without significant Allied help.
The entry of the USSR was, IMHO, probably just the coup de grace for a country already facing a number of strategic disadvantages. Cheers.