colourbleu - 6:30 pm on Jul 25, 2010 (gmt 0)
Google record the date found of Back links as an when found. Google will also use this information when dating the age of a domain for example: a domain is only as old as the oldest link. If its been around but parked for 10 years and only got a proper page a year ago and a single back link at that time it would only be considered a year old.
BLs have an effect at time found, but if they are super Juicy (say a PR6 with no other outbound links), that juice will be gradually passed over maybe 2 years.
In fact such a powerful link may not actually help ranks at all to start with nor pass any juice because it is so out of keeping with the natural links of the domain.
I suspect Google also never forget you had a link, while it may have diapered, they most likely then use this data to count the link churn you have had and this will no doubt be used as a factor when judging your Trust Rank.