Rackspace announced yesterday, as part of a general unveiling of its cloud strategy, a new partnership with Limelight Networks.

Under the new partnership, customers of Rackspace’s Cloud Files (formerly CloudFS) service — essentially, a competitor to Amazon S3 — will be able to choose to publish and deliver their files via Limelight’s CDN. Essentially, this will place Rackspace/Limelight in direct competition with Amazon’s forthcoming S3 CDN.

CDN delivery won’t cost Cloud Files customers any more than Rackspace’s normal bandwidth costs for Cloud Files. Currently, that’s $0.22/GB for the first 5 TB, scaling down to $0.15/GB for volumes above 50 TB. Amazon S3, by comparison, is $0.17/GB for the first 10 TB, down to $0.10/GB for volumes over 150 TB; we don’t yet know what its CDN upcharge, if any, will be. As another reference point, Internap resold via SoftLayer is $0.20/GB, so we can probably take that as a reasonable benchmark for the base entry cost of CDN services sold without any commit.

It’s a reasonably safe bet that Limelight’s CDN is going to deliver better performance than Amazon’s S3 CDN, given its broader footprint and peering relationships, so the usual question of, “What’s the business value of performance?” will apply.

It’s a smart move on Rackspace’s part, and an easy way into a CDN upsell strategy for its regular base of hosting customers, too. And it’s a good way for Limelight to pre-emptively compete against the Amazon S3 CDN.