When comparing gold to other metals that are considered precious, there does seem to be a pretty large discrepancy. Silver would be in a pretty fair relation to gold in terms of its price-'preciousness' ratio (somewhat undervalued), but when you look at the PGMs (platinum group metals) the picture is different.
In 2007, the world produced 78.7M Oz gold compared to only 7.1M Oz palladium, 6.6M Oz platinum and 824,000 Oz rhodium. Price per Oz in same order (approx.): 930 - 200 - 1000 - 1000. Obviously there is a huge price difference here in relation to production, and these are all considered precious metals, which means that they all are very similar - for example, they don't oxidize particularly easily.
So why the big differences? All of the PGMs had about 50% of their demand coming from the auto-industry for catalysts (in 2007), and we all know what's been happening there lately. In the summer of 2008, rhodium was priced at ~$10,000, while the price-changes for palladium and platinum were not as dramatic, but still a lot larger than for gold or silver.
It's generally considered to be because of the (previously) growing auto-industry that the prices of PGMs went up so much, for example rhodium was at only $200 in 2001. It wasn't the only reason however, the other thing that made prices rise was the trouble of power-generation in South Africa. Today, South Africa has ~70% of world production of PGMs. The electrical power shortage in the country is a long term problem because the country has a monopoly that opposes private initiatives and then there's also the black empowerment laws that sometimes puts poorly qualified people in charge (which is clearly related to an otherwise large shortage of skilled labor in the country).
Apart from the auto-industry, there is a pretty large portion of demand coming from other industrial demand, as well as jewellery (less than 20% on average); the demand for PGMs as investments are, although clearly existing, very small.
If the PGMs really are precious, shouldn't they be just as interesting for crisis-investment as gold or silver? I think that the main reason for the firmer position of gold and silver would be the fact that they've been used as money in the past, and might be so again, and also the much easier accessibility of gold and silver. And by the looks of it today, much of the PGM-prices are ruled by the assumption that there will be a large shift toward electric cars in the coming years and that the problems in South Africa will not be large enough to work against the fall in demand.
I suppose in the end, if you want to somehow invest in PGMs, it would be on a more ideologically motivated basis - because they are more precious than gold or silver, no doubt about it - but if only a very small portion of humanity actually sees them as truly precious, then they will be ruled by industrial demand. I guess the most important lesson to take home from this comparison of gold-silver/PGMs is that gold could be considered high priced on a preciousness/price ratio, but considered as a potential future currency it would have to be compared to the preciousness of pieces of paper, which would be about infinite to zero. If gold will be used as a currency, then there could be a slight chance that someone would want to use the PGMs as well... which would mean humongous price-appreciation. I wouldn't bet on it, but the prospect would definitely represent one of the largest gains ever if it were realized.