Can I include the text in another cell to identify a data source ?

Hello all, 

Bit of a left field request. 

I have a database with several sheets and want the user to be able to compare data in two of them. 

To get the cell A1 data from the second sheet, I'd normally just enter: =Sheet1!A1

(it's actually several columns and hundreds of rows of data, but this is adaquate to explain my query)

However for this database, I need the user to be able to select which two tabs they want to compare. So what I need to know is; can I get them to enter the identity of a sheet in cell B1, then get the look up instruction of cell C1 to use that text within B1 ?

ie, something similar to these (which don't work): =Sheet(B1)!A1 , or;  =B1!A1

Hope that explains what I'm in need of, quite difficult to put it into words.. lol