First time getting "Service Engine Soon" light -- Help --

[Critical Drew]

I've had my 2003 4.4i X5 for about 2 years now. With the exception of an alternator replacement I haven't had any running or driving problems. The other evening I saw that my "Service engine soon" light had come on. The vehicle seemed to be running and driving normally and nothing was displayed on the information screen in the cluster.
I scanned the computer the next day and found that it was an error for Engine running too lean at idle, Bank 2.
I cleared the code and went on my way. 2 days later the light is back.

Is anyone familiar with this fault and its usual diagnosis?
The vehicle cracked 100,000mi almost 1 week prior to this light coming on, but as I said earlier everything is operating normally.
Where should I start looking for the culprit(s)?