A fertility specialist wrote my wife a prescription for synthroid (thyroxine) for a tsh of 2.9, borderline. previous test was 2.1. what about retesting first?

Yes: It's reasonable to retest if there are no symptoms, may ask for free T3 (liothyronine) and free T4 which will give more of a picture of thyroid status.

TSH above 2.5: For women that are pregnant, studies indicate an increase in miscarriage rates in women with TSH of 2.5-4.5. http://bit.ly/xfcrsy this data is preliminary and needs further study, but in the meantime the risk of supplementation with thyroid hormone is small, with potential benefit significant. Some obstetricians are now recommending supplementation of thyroid hormone in these women.

More than 1 approach: As she is trying to conceive, and a "normal" TSH in early pregnancy is under 2.5, it's reasonable to treat with synthroid (thyroxine). If retesting instead i would check thyroid hormones and thyroid antibodies to decide if treatment is needed. If you undergo ivf i would treat as elevated estrogen levels increase thyroid hormone binding protein and raises tsh. Untreated women may get more miscarriages with ivf.