what happens if a husband and wife are both participants in the plan and the husband dies. the surviving spouse is the husbands sole beneficiary. does the plan allow the accounts to be combined under one account or should they be kept separately as long as they have not been paid out?

The beneficiary account has different rights and features that must be tracked, however you may accomplish that.

with the spousal rollover rules aren't the accounts the same at this point?

...but the benefit was not rolled over.

true. so the account benefit attributable to the deceased husband must be dealt with in accordance with 401(a)(9) or rolled over?

Yes. And keep in mind that among the rights and features of beneficiaries that QDROphile was referring to are an exception to the 10% early withdrawal penalty and different withdrawal restrictions. So on the off chance that the spouse might need access to the money, it might be better done from a beneficiary account (depending on her specific facts and circumstances).