As far as I know, your last 07 payroll is the cutoff date. There aren't any salary deferrals from the previous year(07) made in the current year(08), unless those contributions were recorded with an effective date in the previous year. They are then treated as receivables for the plan in the current year. Typically, these are the result of 12/31 payrolls which are effective 12/31, but the contributions were received by the Pension Administrators in the current year. There might be instances in which contributions are made to your plan in 08 for 07, but they are largely employer contributions and are not 401(k) salary deferrals. These contributions can be of variety of matches (safe harbor, non-elective, or otherwise), profit sharing, money purchase contributions, ect.
You can make an IRA contribution for 2007 when you file your taxes, in April. I dont think you can put it into your work plan, but you can create a regular IRA and get the tax benefit.
