Following a Whois request on a website the result came back as the following:

netnum: 89.234.34.48 - 89.234.34.55
netname: RSPC-UK-Media-Surface
descr: Media-Surface IP Space
country: GB
admin-c: IA247-RIPE
tech-c: IA247-RIPE
remarks: rev-srv: ns.rackspace.com
status: ASSIGNED PA
mnt-by: RSPC-MNT
source: RIPE # Filtered
remarks: rev-srv attribute deprecated by RIPE NCC on 02/09/2009

person: IP Admin
address: Rackspace Managed Hosting
112 E. Pecan St. Suite 600

Can somebody tell me if this means that the site is hosted by Rackspace with the CMS powered by Alerion/Media Surface?

Many thanks

The whois tells you IP range belong to Rackspace.
It can be a server managed by Rackspace or colocated at Rackspace.
IP whois is not related to script used on website.

The whois record just says the ip range belongs to Rackspace, it does not give any insight to what the ips are used for, what type of server or what application.