Carphone Warehouse, the largest independent European mobile phone retailer and an official iPhone reseller, added four placeholders to their inventory list for new iPhone models earlier this week, sources report.

With new iPhone models expected this summer, in addition to the previously-announced 3.0 firmware, Carphone Warehouse appears to be gearing up for the change. A screenshot of its inventory system provided by a person known to have access to such information shows placeholders for both 16GB and 32GB iPhones in both white and black. They’re mixed in among the existing and previous iPhone models, having reportedly cropped up just hours ago.

Numerous reports and inadvertent

leaks from Apple’s partners over the past several weeks have signaled the arrival of third-generation iPhones in capacities of 16GB and 32GB. While the changes to Carphone Warehouse’s systems should not be treated as confirmation of new iPhone specifications, the retailer’s status as an official iPhone reseller could potentially make it privy to such information.

Coupled with Best Buy’s recently-released internal memorandum notifying employees of expected low inventory of the current iPhone 3G we reported, as well as a similar advisory issued by Brightpoint, an iPhone distributor in Australia, retail signs are aligned in pointing to new iPhone disclosures early next week at Apple’s annual developers conference.