Photo quality aside, it seems that Canon DSLRs are more popular than Nikon DSLRs right now based on Flickr stats.
Below is a ranking of popularity based on daily users (taken from Flickr.com)
Canon Camera (Average daily users)
EOS 5D Mark II (4350) $2500
EOS REBEL T2i / 550D (3908) $500-$600
EOS 7D (3577) $1500-$1800
EOS REBEL T1i / 500D (2507) $400-$500
EOS 60D (2219) $800-$1300
Nikon Camera (Average daily users)
D90 (3985) $1000-$1200
D7000 (2664) $1200-$1500
D3100 (2173) $500-$700
D700 (1912) $2200-$3000
D5000 (1521) $600-$1000 (superseded by D5100)
Key to below stats:
# of items - This is the total number of photos uploaded per model.
Avg. daily users - This is the number of Flickr members who uploaded 1 or more photos yesterday.
Activity Factor The "Activity Factor" is a number derived from photos, members and a model's rank to indicate cameras that are used "a lot".
Type Point & Shoot / DSLR / Cameraphone / Camcorder
Rank A model's rank (within its brand) is based on how many people used it yesterday.
Links for more info.