Well there was definitely a slip in sales, and not just in the USA.
From "Peeling off those dollar bills", here's estimated UK sales for U2 albums.
1. The Joshua Tree : ~2,670,000 (2,665,553 as at 22nd Oct 2006)
2. Best of 80-90 (both versions) : ~1,925,000 (1,919,678 as at Sat 25th Nov 2006)
3. Rattle & Hum : ~1,400,000 (~1.23m by Oct '94)
4. Achtung Baby : ~1,350,000 (1.13m by Jan '98)
5. UABRS : ~1,300,000 (3xP by April 1987 & ~1.1m by end of '80's)
6. HTDAAB : ~1,240,000 (1.2m by end of 2005)
7. ATYCLB : ~1,130,000 (March 12th 2005 edition of Musicweek said 1,083,169 & has since done ok in top 100/200)
8. The Unforgettable Fire : ~890,000 (~810k by end of '80's)
9. War : ~830,000 (~750k by end of '80's)
10. Best of 90-00 (both versions) : ~875,000 (871,765 as at Sat 25th Nov 2006)
11. U218 Singles : ~620,000
12. Zooropa : ~600,000 (~500k in '93)
13. Pop : ~450,000 (370k in '97)
14. October : ~400,000
15. Boy : ~300,000
16. WAIA : ~150,000