europa,
a fine question. thoughts on this vary. i'll start with the american perspective, since that's what i'm most familiar with.
some people believe that you should overweight your home country because that's where you live and that's where you'll be drawing down your retirement savings in the future. some also believe that the us is in fact a "special case" since it is the only world superpower left and since the global economy is intimately tied to what the us is doing anyway.
there are others who see this kind of home country bias as too risky. much as how owning your company's stock is poor diversification (the kinds of events that affect the stock price correlate strongly with the kinds of events that affect your paycheck and continued employment), they argue that you want to keep more eggs outside your home basket.
vanguard's TR funds only hold 30% of equities in international. they have
a paper where they justify this number. basically, international markets are more volatile than the us market, and they calculated that the best risk-adjusted roi ("bang for the buck") happens around 70/30 us/intl.
i don't know who's right, so i look at it pragmatically: the total market weight of us stocks vs intl stocks is about 45/55[1], and 50/50 is very simple to manage. hence, i go 50/50 us/intl.
now, what about non-americans?
most folks i've seen do weight their home country a little extra. i think the arguments pro and con are basically the same as what i've laid out above.
there's an article in the bogleheads wiki about this but i can't find it right now.
[1] in fact, if you like the market weight solution, you can simply purchase Vanguard Total World Index (VTWSX or VT) and let them handle it. the downsides are a slightly higher ER, less flexibility, and the inability to concentrate international holdings in taxable space to take maximum advantage of the (small) foreign tax credit.