.htaccessに以下を記述し、携帯端末のみ閲覧可能なページにしています。

リストアップされているキャリアのIP帯に関しては他の人の回答に任せるとして(^^;

一般的ないわゆる携帯端末と言って良いか分かりませんが、ソフトバンクのiPhone3Gのネットワークが無いようなので書いときます。

# Softbank iPhone 3G
allow from .panda-world.ne.jp
126.240.0.0/12

iPod TouchやiPhoneの扱いはどうなっていますか？

http://sourceforge.jp/magazine/08/10/31/0839204

他の方の回答がないようなので、現在うちで使っているリストを貼っておきますね。

ちなみに逆引きホスト名はいくらでも詐称可能だしWEBサーバのパフォーマンスにも影響するので使ってません。

# DoCoMo
allow from 124.146.174.0/24
allow from 124.146.175.0/24
allow from 203.138.180.0/24
allow from 203.138.181.0/24
allow from 203.138.203.0/24
allow from 210.136.161.0/24
allow from 210.153.84.0/24
allow from 210.153.86.0/24
allow from 210.153.87.0/24

# Softbank
allow from 123.108.236.0/24
allow from 123.108.237.0/27
allow from 202.179.204.0/24
allow from 202.253.96.224/27
allow from 210.146.60.192/26
allow from 210.146.7.192/26
allow from 210.151.9.128/26
allow from 210.169.130.112/28
allow from 210.175.1.128/25
allow from 210.228.189.0/24
allow from 211.8.159.128/25

# Softbank iPhone3G
allow from 126.240.0.0/12

# AU
allow from 59.135.38.128/25
allow from 61.117.0.128/25
allow from 61.117.1.0/28
allow from 61.117.1.128/25
allow from 61.117.2.32/29
allow from 61.117.2.40/29
allow from 61.202.3.64/28
allow from 118.152.214.192/26
allow from 118.159.131.0/25
allow from 118.159.133.0/25
allow from 121.111.227.0/25
allow from 121.111.227.160/27
allow from 121.111.231.0/25
allow from 121.111.231.160/27
allow from 210.230.128.224/28
allow from 218.222.1.0/25
allow from 218.222.1.128/28
allow from 218.222.1.144/28
allow from 218.222.1.160/28
allow from 219.108.157.0/25
allow from 219.108.158.0/27
allow from 219.108.158.40/29
allow from 219.125.145.0/25
allow from 219.125.146.0/28
allow from 219.125.148.0/25
allow from 219.125.148.160/27
allow from 219.125.148.192/27
allow from 219.125.151.128/27
allow from 219.125.151.160/27
allow from 219.125.151.192/27
allow from 222.5.62.128/25
allow from 222.5.63.0/25
allow from 222.5.63.128/25
allow from 222.7.56.0/27
allow from 222.7.56.128/27
allow from 222.7.56.192/27
allow from 222.7.56.224/27
allow from 222.7.56.32/27
allow from 222.7.56.96/27
allow from 222.7.57.128/27
allow from 222.7.57.160/27
allow from 222.7.57.192/27
allow from 222.7.57.224/27
allow from 222.7.57.32/27
allow from 222.7.57.64/27
allow from 222.7.57.96/27

# AirH
allow from 61.198.128.0/24
allow from 61.198.129.0/24
allow from 61.198.130.0/24
allow from 61.198.131.0/24
allow from 61.198.132.0/24
allow from 61.198.133.0/24
allow from 61.198.135.0/24
allow from 61.198.136.0/24
allow from 61.198.137.0/24
allow from 61.198.138.100/32
allow from 61.198.138.101/32
allow from 61.198.138.102/32
allow from 61.198.138.103/32
allow from 61.198.139.0/29
allow from 61.198.139.128/27
allow from 61.198.139.160/28
allow from 61.198.140.0/24
allow from 61.198.141.0/24
allow from 61.198.142.0/24
allow from 61.198.143.0/24
allow from 61.198.160.0/24
allow from 61.198.161.0/24
allow from 61.198.162.0/24
allow from 61.198.163.0/24
allow from 61.198.164.0/24
allow from 61.198.165.0/24
allow from 61.198.166.0/24
allow from 61.198.168.0/24
allow from 61.198.169.0/24
allow from 61.198.170.0/24
allow from 61.198.171.0/24
allow from 61.198.172.0/24
allow from 61.198.173.0/24
allow from 61.198.174.0/24
allow from 61.198.175.0/24
allow from 61.198.248.0/24
allow from 61.198.249.0/24
allow from 61.198.250.0/24
allow from 61.198.251.0/24
allow from 61.198.252.0/24
allow from 61.198.253.0/24
allow from 61.198.254.0/24
allow from 61.198.255.0/24
allow from 61.204.0.0/24
allow from 61.204.2.0/24
allow from 61.204.3.0/25
allow from 61.204.3.128/25
allow from 61.204.4.0/24
allow from 61.204.5.0/24
allow from 61.204.6.0/25
allow from 61.204.6.128/25
allow from 61.204.7.0/25
allow from 61.204.92.0/24
allow from 61.204.93.0/24
allow from 61.204.94.0/24
allow from 61.204.95.0/24
allow from 125.28.0.0/24
allow from 125.28.1.0/24
allow from 125.28.11.0/24
allow from 125.28.12.0/24
allow from 125.28.13.0/24
allow from 125.28.14.0/24
allow from 125.28.15.0/24
allow from 125.28.16.0/24
allow from 125.28.17.0/24
allow from 125.28.2.0/24
allow from 125.28.3.0/24
allow from 125.28.4.0/24
allow from 125.28.5.0/24
allow from 125.28.6.0/24
allow from 125.28.7.0/24
allow from 125.28.8.0/24
allow from 210.168.246.0/24
allow from 210.168.247.0/24
allow from 210.169.92.0/24
allow from 210.169.93.0/24
allow from 210.169.94.0/24
allow from 210.169.95.0/24
allow from 210.169.96.0/24
allow from 210.169.97.0/24
allow from 210.169.98.0/24
allow from 210.169.99.0/24
allow from 211.126.192.128/25
allow from 211.18.232.0/24
allow from 211.18.233.0/24
allow from 211.18.234.0/24
allow from 211.18.235.0/24
allow from 211.18.236.0/24
allow from 211.18.237.0/24
allow from 211.18.238.0/24
allow from 211.18.239.0/24
allow from 219.108.0.0/24
allow from 219.108.1.0/24
allow from 219.108.10.0/24
allow from 219.108.11.0/24
allow from 219.108.12.0/24
allow from 219.108.14.0/24
allow from 219.108.15.0/24
allow from 219.108.2.0/24
allow from 219.108.3.0/24
allow from 219.108.4.0/24
allow from 219.108.5.0/24
allow from 219.108.6.0/24
allow from 219.108.7.0/24
allow from 219.108.8.0/24
allow from 219.108.9.0/24
allow from 221.119.0.0/24
allow from 221.119.1.0/24
allow from 221.119.2.0/24
allow from 221.119.3.0/24
allow from 221.119.4.0/24
allow from 221.119.5.0/24
allow from 221.119.6.0/24
allow from 221.119.7.0/24
allow from 221.119.8.0/24
allow from 221.119.9.0/24