Shaun Rogers was traded to the Bengals for a 3rd and 5th rounder, which was rescinded and then he was finally dealt to the Browns for Leigh Bodden. My guess (and it's purely a guess), is that after the trade was agreed to, Cincinnati was either under the impression or was going to wait until after midnight (when the Lions would have been responsible for Rogers roster bonus) to submit the trade proposal to the league. Millen balked at the idea that he'd pay the roster bonus and traded Rogers to the Browns. Which is ok, I personally like that trade better. It's purely a guess on my part.
Breaking down Shaun Rogers contract
In December of 2004, Shaun Rogers signed an extension. When he signed the extension, he received a $9M signing bonus. In March of 2005, he received a 3.5M guaranteed roster bonus. The contract called for him to receive base salaries of $650K for 2005, $850K for 2006, $1.5M for 2007, $4.25M for 2008, $5.25M for 2009 and $7M in 2010. He was also due $1M roster bonuses (non-guaranteed) on the first of March in 2008, 2009 and 2010.
When a player receives a signing bonus, the CBA allows the signing bonus to be amortized over the length of the contract. However, for the 2004 season the maximum proration for signing bonuses to be amortized was six years. So, the Signing bonus is allocated proportionately over the first 6 years of the contract (2004-2009) and nothing is allocated to the final year of the contract. $1.5M allocated to each year through 2009.
The guaranteed roster bonus, for salary cap purposes, is treated just like a signing bonus. The $3.5M is amortized over the length of the contract. For the 2005 season (the year the bonus was paid), the maximum proration of signing bonuses was 5 years. The roster bonus would have been amortized proportionately over the five years from 2005-2009 or $700K per year.
According to Rogers contract, his salary cap figure by year, would have been as follows:
2004 $2,213,750 (1.5M added to his current 2004 cap figure of $713,750)
2005 $2.85M (650K in salary, 1.5M in SB amortization and 700K in RB amortization)
2006 $3.05M (850K in salary, 1.5M in SB amortization and 700K in RB amortization)
2007 $3.7M (1.5M in salary, 1.5M in SB amortization and 700K in RB amortization)
2008 $7.45M (4.25M in salary, 1.5M in SB amort, 700K in RB amort, 1M in RB)
2009 $8.45M (5.25M in salary, 1.5M in SB amort, 700K in RB amort, 1M in RB)
2010 $8M (7M in salary and 1M in RB)
In 2008, Shaun Rogers would have been included in the Lions cap calculation for 7.45M, when he was traded, we would have saved his salary (4.25M) and his roster bonus (1.M), but we would also have to accelerate the remaining unamortized portion of his bonuses (1.5M and 700K) that were allocated to 2009. So the cap effect of trading Rogers was $3.05M cap savings). His cap figure on our books for 2008 would be 4.4M (2.2M in 2008 amortization and 2.2M in accelerated 2009 amortization), commonly referred to as dead cap.
Leigh Bodden was acquired in the trade of Rogers. He is due base salaries of 1.7M in 2008 and 1.8M in 2009. Current year Cap cost $1.7M.
Assuming that Adam Schefter was correct when he reported that the Lions were 6.8M under the cap on 02/21/08. And since he's looking at actual reports generated by the league, there's no reason to assume that he isn't correct. From now to the start of the season, teams only must count the top 51 player cap costs toward their cap figure. So, by my estimation, we are about 8.5M under the cap.
Askthecommish.com has a nice FAQ section that explains a lot of the rules of the salary cap and how it relates to player contracts. http://www.askthecommish.com/salarycap/faq.asp