To take the example from the docs:

AggregateResult[] groupedResults = [SELECT CampaignId, AVG(Amount) FROM Opportunity 
GROUP BY CampaignId];

Now try

Map<ID, AggregateResult> resultsMap = [SELECT CampaignId, AVG(Amount) 
 FROM Opportunity 
 GROUP BY CampaignId]; 

This won't work even though CampaignId is of type ID. To make it work, alias CampaignId with Id:

Map<ID, AggregateResult> resultsMap = [SELECT CampaignId Id, AVG(Amount) 
 FROM Opportunity 
 GROUP BY CampaignId]; 

Interestingly, the case here does matter. Aliasing with id or ID won't work.

This worked on March 28th 2012 (seriously, I swear it did! :-), but does not any more.

Please support my idea at https://sites.secure.force.com/success/ideaView?id=08730000000hmhOAAQ to reinstate this behavior.