If those two SIM cards works with other phones just not with N900, I'd check dmesg and maybe even lsmod from N900. This is a far shot (I know because I have two N900, one with SIM card and is connected to the cellular network and the other one doesn't have SIM card and has no cellular connectivity), but it may show information as to why it is not connected to the cellular network.

Here is my pastebin output of one N900 connected to a cellular network and the other not connected at all. The first column is for N900 connected to a cellular network and the second column is for N900 not connected to any cellular network: http://pastebin.com/3r94eKp7 . Judging from the output, the missing modules seems to note that there's no significant difference (i.e. that one has cellular module loaded and the other one doesn't) between the two. I'm presuming the module that deals with connecting to cellular network is loaded regardless whether or not you have a SIM card inserted into the N900?