Hi all, been a while since I've been here! It's been a bit quiet on the 'keitais outside Japan' front so I haven't been compelled to come often...
But anyway, as nobody has worked out what SoftBank's triangle (△) display means in their network checker, I'm posting here.
The triangle typically displays for handsets that still have money owing on them, meaning they were purchased on contract without paying the full price upfront (ie paying up to 2000 yen/month for the handset alone), and the owner who purchased it hasn't finished repaying the handset's cost over the full two years of the contract yet.
The triangle description text says "There is the possibility this handset may be locked in future." This is written because if the handset owner disappears from SoftBank's network without paying out their handset contract, SoftBank will Red-ROM the phone as a consequence.
I've worked all this out from looking for SoftBank handsets at Yahoo Auctions. A lot of the newer handsets (940SH etc) being sold there are marked in the network checker as a triangle because their owners are selling them early after upgrading to an iPhone or such (and they offer guarantees of refunds if the phones get locked because they're still responsible for paying out the phone contract amounts).
So yeah, if there's a triangle on the phone you bought/own, you'll want to hope the previous owner keeps up their phone repayments, or it may get Red-ROMed at some point in the future (although if you never intend to use the phone on SoftBank's network then it's a moot point).