The International Association for Dental Research conveys the latest warning (.pdf):
Numerous case reports and a few clinical studies have examined the association between tongue piercing and receding gums, and oral piercing, typically involving the tongue and/or the lip, has been associated with periodontal complications. However, there are no such studies on lip piercing.
A research group from the Ohio State University in Columbus recruited, from the community, 29 young adults with lip piercing (mean age, 21 years, 8 months; 15 females) and 29 without (mean age, 22 years, 3 months; 15 females) and examined them for gingival recession on the buccal aspect of the mandibular central incisors.
There were no demographic (age, gender distribution) differences between the two groups of subjects. Receding gums were were significantly greater among subjects with lip piercing (41.4%) than among controls (6.9%). The average recession depth was more than double in subjects with piercing compared with controls. Length of time of wear was the only significantly associated demographic variable.
These results indicate that lip piercing is strongly associated with increased prevalence and severity of receding gums, and increased time of wear is associated with increased prevalence of recession.