Really? I had a look at exactly what swappolube was doing when it was first released, and simply put it was tuning some kernel parameters, none of which could IMO cause any harm to the N900. One significant one was tuning swappiness from 100 (the maximum) to 0 (the minimum), this means the kernel will try to use swap as little as possible instead preferring to drop cache from RAM, whether this helps or hurts performance depends on whether you are more likely to need the program data or the data in the cache because it can sometime be beneficial to swap out unused programs and have a greater amount of cache in RAM with the cache saving extra reads from the disk. This tuning will in fact reduce the amount of writes to the swap partition, which reduces the wear on the flash memory chips.

There was only one option I was slightly concerned about, and that was the one turning off TCP timestamps, I didn't think there was any real benefit to doing it, and it could potentially cause problems with you network connections, but even this wouldn't actually harm your N900 and can easily be changed back.

Now, I haven't looked at what changes have been made to swappolube since the first version, I uninstalled it to test something then just reinstated the changes it made without reinstalling it, so I haven't seen the newer versions, but from what it was when I did look at it is was mostly harmless, and I wouldn't worry about it causing any harm or bricking my N900.

I'm also still on PR1.2 (cause I'm on the UK firmware), so I haven't seem first hand what changes PR1.3 made, but I think the most likely result of installing it would be not quite as noticeable as it was with PR1.2 because there is less margin for noticing an improvement.

EDIT: I just checked the swappolube-nogui from extras-testing and it seems to be just the same with its tweaks. I also made a little mistake in my post, swappolube changes swappiness from 100 to 30, not 0. I wouldn't hesitate to say it is safe to use. I haven't check the regular version with the GUI, but that should be just as safe, just with the addition of a GUI to change the tuned parameters to your liking.