Disclaimer: This does not constitute tax advice. Consult a CPA.
Capital Asset vs. Amortization
Historically, the IRS has treated domain names as capital assets (Section 197 intangibles). This means the cost of acquiring a premium domain is typically amortized over 15 years, rather than deducted entirely as an immediate expense in the year of purchase.
However, if the domain is purchased as part of an entire operating business (vs. just the naked domain), different allocation rules may apply.