Yes, there are several IRS letter rulings allowing the surviving spouse who is the sole estate beneficiary to roll over the proceeds to her own IRA. Following is a link to Bruce Steiner's 1997 article on the subject, and the article still remains applicable. Some complications have been added by the 5 year delay in this case:
http://www.kkwc.com/docs/AR20050125164755.pdfIf the proceeds are paid to the estate, they would generally not be considered eligible rollover distributions for 20% withholding purposes. The estate executor could then distribute the IRA proceeds to the surviving spouse who could complete the rollover. This should be done within 60 days of the 401k distribution, but it might take some searching to find an IRA custodian willing to open the rollover IRA.
Another hurdle is that the RMDs required of your mother as a beneficiary cannot be rolled over to her own IRA. Those amounts should be distributed to her and she should file a 5329 requesting a waiver of the excess accumlation penalty. Remember that there was NO RMD required for 2009.
Most plans will refuse the direct rollover to her IRA even if provided with references from Bruce's article, but she could always ask. Again, the life expectancy RMDs complicate the situation now that 5 years have passed. If your father died prior to his required beginning date with an estate beneficiary, the 5 year rule would apply and that is probably why they use the end of the 5th year after his death as a deadline for distribution.
If Bruce sees this, perhaps he will comment on the most efficient way to proceed with the plan and/or IRA custodians assuming she wants to preserve her rollover at this point in time.