Im from the Uk and yes the Euro Rate will make me watch what i spend during my time on the island, but you have to remember this is Ibiza and if you cant handle that then don't come.
But....
i)A large percentage of people who go to the clubs are from the UK, that is a fact
ii) As many people will tell you as a whole the clubs were generally quieter in 2008 than previous years
the two points above should see a slight thawing of the crazy drink prices, don't expect them to become reasonable, all I'm saying is they "might" drop down a Euro or so.
For me if things stay as they are I will be using the busses and forgetting taxi's , drinking much much less in the clubs but still enjoy myself
There is a school of thought that by the summer the interest rate cuts will have filtered down, the VAT cut will have helped things, and the actual demand for people buying Euro's will help bolster the pound.
only time will tell.