The 3400/3500 series was used in all the Rio flash players from the S10 on, as well as Nitrus and Carbon. It's Sigmatel's bread and butter for portable devices, so I'm sure they were interested in the software Rio made for the 3500 just as much as Rio's ARM software.
I do think it's likely that we'll see several other manufacturers release 3500 devices with Rio's software on them, although I have no way of knowing if the MobiBlu device is one of them. I, for one, am only interested in the players with the 3600 and the empeg software, if and when one might be available to purchase. I will buy the first one I see, assuming that it doesn't have some stupid crippling limitation or cost >$1000.