Current Allendales are infact failed conroe's with 2megs of the cache failed. Unfortunutly you cant unlock the cache!
This does give the die a higher surface area / power output than the conroe's which is why they tend to overclock abit further.
There will be true 2 meg allendales coming soon which will be cheaper chips and probably end up being the new celerons.
When the core 2 originally came out and they had engineering samples of chips, several sites did comparisons between 2 and 4 meg chips to find where the cache mattered, and superpi was one of those cases.
Edit - Here is one of those reviews for where cache matters,
http://www.anandtech.com/cpuchipsets...spx?i=2795&p=4