OK, so I've sat down and worked out the split year treatment as well. This may be of interest to others as there are often questions like "I'm leaving Germany next May, can I claim back taxes?"
Well the short answer is "yes", but it's not that simple, nor as generous as you might hope. See my previous post above for general allowances. This post is just concerned with tax back if leaving in the middle of a year.
As an example, lets say you're single and were on a salary of €60,000 per year. You leave Germany on 30 June, so actually only earned 30,000 that year. I'll ignore allowances to make it simple and assume that this sum is your "taxable income" (zu versteuendes Einkommen - zvE). Now according to the Grundtabelle 2004, someone on €60,000 should pay €18,555 tax, or 30.9%. Someone who earned €30,000 should pay €6,059 tax, or 20.2%.
So, you're probably thinking that you were taxed at 30.1% on the basis that you would be earning for the whole year, but seeing as you've only earned for half the year, you should have only be taxed at 20.2% and were charged 9.9% too much tax on €30,000. I.e. tax back of €2,970.
However, the Finanzamt uses something called "Progressionsvorbehalt" to work out what percentage to tax your German income at. This is dependant on your foreign income for the year. So let's presume you are going back to a country which Germany has a double taxation agreement with (I imagine this includes all english speaking countries). So you take a few months holidays, and start your new job there in September and earn €20,000 in the last quarter of the year. Based on the double taxation agreement, you can't be taxed on this income in Germany as it was already taxed in your home country.
So the Finanzamt instead adds your foreign income to your German income to work out what percentage tax you should have paid on your German income. So in this case, your total income is €50,000. According to the Grundtabelle, you should have paid €13,667 or 27.3% tax on this income, had it all been in Germany. The Finanzamt then uses this figure of 27.3% to calculate the tax you should have paid on your German income. So you were really only charged 2.8% too much tax, rather than the 9.9% that you assumed. So your tax back would only be €840, not €2,970. This is Progressionsvorbehalt.
Now if your monthly income abroad was actually higher than your German monthly income, you may find that the Finanzamt claims that you owe them money !!! In our example, say you start work straight away in your new country and earn €40,000 from July to December, you may find that the Finanzamt is looking for more money for you as taxes.
Note that I'm not sure if they include all your foreign income, or just the part of your foreign income that is also taxable in that country (i.e. they take your tax free allowances in your home country into account). Also they will give you allowances for the standard stuff as mentioned in the previous post. But I would say that if you earn proportionally significantly more in your home country after leaving Germany, don't bother claiming tax back. You may be presented with a bill !!!
The Bundesfinanzhof (BHF) has ruled that Progressionsvorbehalt is legal even if it is not explicitly mentioned in the double taxation agreement between the two countries involved - Urteil vom 19.12.2001 (I R 63/00, BFH/NV 2002, S. 584). The case involved a German who moved to the US during a calander year. It's not clear if this ruling is valid if you move within the EU, though the Finanzamt is already using it (they used it in my case when I moved from Ireland to Germany). There are open cases in Europe concerning this, so it may be possible to appeal your Steuerbescheid once these cases are decided. See here
Now I guess you could tell the Finanzamt that you're leaving Germany to study, or go on holidays, and have no foreign income for the remainder of the year. In that case you should get much more tax back. But I've no idea whether the Finanzamt would ask for further proof of this, or could otherwise check whether this claim was true.
Pretty long winded and complicated, but hopefully it will help.