I don't really agree with the previous statements. They are either incorrect or unclear.
3G requires an uplink and downlink, usually on seperate bands, but some providers just use a single band.
3.5G = High-Speed Downlink Packet Access, this means the n900 had a 3G connection and has temporarily established additional data channels.
n900 does not support MIMO or dual-cell HSDPA, but future phones supporting dual cell will be using two seperate frequency bands to double the bandwidth of the downlink. MIMO just connects with two antennas to the same band.
Anyways, if the uplink on the n900 is on a seperate band, and interference causes it to be lost, the phone will probably drop to 2.5G, if available. Actually it's hopefully EDGE (2.75G) in most areas, n900 doesn't display 2.75G though, and i'm not sure if it has an edge symbol on the display, but anyways n900 definately supports GSM frequencies so it should work on some providers.