If anyone is using Bitcoin, the distributed digital currency, you can now run the Bitcoin server on your N900, thanks to a port by user doublec at the Bitcoin forum.

You can read about it and download the executable from here:
http://www.bitcoin.org/smf/index.php?topic=2125

This is the non-gui version of Bitcoin (the GUI version uses the wxWidgets library and will be harder to port). Here's how it looks:

loving my N900 sooo much due to this very kind of thing...thanks for finding it!

how much battery charge is lost by leaving this running in the background?

I only run it when plugged in to the charger or to a USB socket. But I'm one of those who has never been able to get a full day out of the N900 battery if I leave WiFi on.

Power consumption is very heavy for the first six hours while Bitcoin is downloading the cryptographically-signed block chain. You absolutely can't do that bit on battery power, nor can you do coin generation on battery power.

If you're just doing transactions, the data "chatter" is fairly light and I don't think it would be a battery life dealbreaker.

The page is gone now, but seeing as there is an armel package of bitcoind in Debian, what would porting to fremantle or harmattan take?

Is there a guide on porting from Debian to Maemo? I know of optify, but didn't find much more. (Well, there's scratchbox)

There is no point mining with a N900 even if you could time travel back to the early days and mine the easy hashes, now you wouldn't even mine one block as faster miners would claimjump your blocks every time until the heat death of the universe, your wasted batteries and CPU cycles would just add to the wasted enthropy.
Use this console app as designed for a portable bitcoin wallet, write a QT app if you want a nice GUI frontend.
If you want to get fancy get a square brand credit card reader and utilize the magread app code to add ability to convert national currency to bitcoin so you can accept a credit card for a mobile point of sale.

I don't think anyone is seriously considering to use their phones to mine for bitcoin.

But as a mobile payment tool, I'd love it.

A silly calculation.
Based on http://blockexplorer.com/q/nethash - I make the difficulty at the time of release about 600, rising to 1200 a month later.
http://www.alloscomp.com/bitcoin/calculator

At current prices - if you had started bitcoind when the original release was posted in this thread on your n900, and then left it running for about 3 months, you would have around 3 bitcoins likely by now, worth about $3000.

Hey! Just running bitcoind can earn you bitcoins? Without mining?

No. You need to mine to earn bitcoin. But still, as said above, don't bother; even in shared mining pools and even if you have free (to you) power, you'll still not make enough to make it worth it, and probably shorten the life of your hardware noticably.