The FCC today released a status update for number portability complaints [download, PDF]. Overall complaints are fairly low, but AT&T Wireless has a large lead:

• AT&T Wireless - 2787
• Sprint PCS - 1445
• Verizon Wireless - 933
• T-Mobile - 854
• Cingular Wireless - 849
• Nextel - 420

It should be noted that many times, a complaint is lodged against two carriers by one customer (e.g. a customer having trouble switching from AT&T Wireless to Sprint PCS could file a complaint against both companies).