Yes! You can contribute to both Roth and Traditional IRAs same time provided total of contributions in both IRAs is not more than maximum contribution allowed for any IRA ($4000).
Maximum contribution allowed for any IRA (Traditional or Roth) is $4000 for year 2006/2007 (age < 50 yrs).
If you contribute $2000 in Traditional IRA, then you can contribute $2000 more in Roth IRA same time that year (2006 or 2007).
Note: For age >= 50 years, additional $1000 can be contributed in IRA.