So will my bitcoins be held in Japan or USA in a months time if I do nothing?
If you do nothing (ie. do not accept CoinLab's ToS on MtGox), then your funds are still held by MtGox Co. Ltd. in Japan (bitcoins and currencies).
How do I know if my account is considered a US/CA account or not? Is this determination based on the IP address used when you opened the account, the source of funding or something else?
We base this on various information, including source of funds and IP address. If you used Dwolla (which is only available to US customers) or if your bank reported you as a US citizen when sending funds to our bank, then you are automatically considered a US citizen.
If you ever connected only from the US, you are also considered a US citizen.
If you believe there was a mistake (ie. when we go live the site asks you to accept CoinLab's ToS), you will be provided an option to specify that you are not a US or CA citizen. Such cases will be reviewed case by case afterward so we can find out what went wrong.
Will the US government have the potential ability to immediately seize any of Mt Gox/Coinlab/Customers cash and/or bitcoins after this transition?
I take it that currently the US would have no jurisdiction to seize assets from a Japanese company, in Japan, but if those assets are now held in the US, by a US company does that not open up that kind of situation?
If the US government were to seize assets from Coinlab in the US, would Mt Gox still cover all customers' balances/deposits (including US customers)?
I do not believe having the funds held in the US would make any easier or more difficult for the US government to seize funds should it find a good reason to do it.
Please note that as you accept CoinLab's ToS, you are in direct contract with CoinLab and no longer with us. In even something would happen with CoinLab (which is unlikely), we would be unable to interfere directly.