It's difficult to analyze the average person's net worth because most of the data comes from informal surveys. In a way, it's like asking people how many times they have sex--chances are, they're going to focus on the good times and give you a higher number.
Anyway, here is one link that leads to a compilation of net worth numbers. CNN's money calculator indicates that most people making $120K a year have a $301K median net worth. That seems high, but perhaps not so unusual for a two-income family on either coast who bought their home twenty years ago.
Bonus: according to EBRI, "More than half (56.4 percent) of those owning at least one IRA had less than $25,000 in them in 2008."
The average IRA account balance in 2008 was $54,864. Because the EBRI IRA database can aggregate multiple accounts held by one individual, the new EBRI analysis also finds the average IRA individual balance (all accounts from the same person combined within the EBRI IRA database) was significantly higher, at $69,498.
The median IRA account balance was considerably less: $15,756 per account and $20,046 per individual. Median levels mark the mid-point (half above and half below) and are less affected by outlier data.
Elsewhere on EBRI, they report that at "year-end 2007, the average account balance in the EBRI/ICI database was $65,454, compared with $61,346 at year-end 2006. 401(k) account balances varied with participant age, tenure, and salary. Individuals with account balances of less than $10,000 were primarily young workers or workers with short job tenures. In contrast, those with account balances in excess of $100,000 were primarily older workers or workers with longer job tenure."
Median 401k balances are harder to come by. According to EBRI and the Investment Company Institute, at the end of 2007, someone in his/her 50s making $60,000 - $80,000 had a median 401k balance of $160,324 and someone in his/her 50s making $80,000 - $100,000 had a median balance of $226,266.
It appears almost all Americans have most of their net worth in their homes.