So our friend Shane from OSBB, did a little investigation work into BlackBerry 10 and BBM. The conclusion, BlackBerry 10 will only support one smartphone at a time connected to BBM.
How was this discovered? The new BBM 7 that is available in the Beta Zone no longer uses your device's PIN, and instead uses your BBID (BlackBerry ID). The story is, that this is the only way to get BBM to work with BlackBerry 10.
I ran into the same problem that Shane did. When I updated to BBM 7 on my Torch 9850, I was also registered under the same BBID on my BlackBerry 10 Dev Alpha device. Therefore, I was forced to transfer any BBM content from my Dev Alpha to my Torch 9850. Now, I can no longer use BBM on my Dev Alpha unless I register it under another BBID.
@BlackBerryDev reported that when BB10 officially launches, "only one smartphone will be able to connect to BBM at a time."
This isn't the best news for those who were hoping to have multiple devices under only one BBM account. But perhaps RIM will figure out a solution in the future for multiple device support.
Something else that should be noted is, how will this apply to the BlackBerry PlayBook when it gets BlackBerry 10? The statement made by @BlackBerryDev stated that only smartphones will be limited. So perhaps tablet/smartphone relationships won't apply.