I've been accumulating the most reliable iPhone 4G specs.  No, you can't hold me to any of these but as rumors go, they're reasonably reliable. Reminder: 4G, if called that, would refer to 'fourth generation' iPhone, not so-called 4GS network. Though I believe TIPB is correct in calling it iPhone HD, especially as this will make Ballmer angry and Jobs chuckle.

• Folders, which are partly auto-managed by iTunes (e.g. it can automatically put all your games in a "GAMES" folder)
• In black and white
• Boxier design
• Micro-SIM slot (no MiniSim, as with current iPhone)
• 16gig, 32gig and 64gig versions
• Alumninum unibody; however, some reliable reports of slighly remade matte aluminium (or ceramic) backside to promote better call reception
• Front facing camera (2mp)
• 5-10 megapixel camera on back (with LED flash); assume 5mp, and with geotagging
• 720p video recording (HD video recording)
• 940 x 640 display
• based on display and rumors, assume 300dpi view and ability to print out screens (not reliable)
• Multitasking (true multitasking including for non-Apple/third-party apps); note, with a front-facing camera and true multitasking this should create a massive multi-billion-dollar opportunity for new location-based start-ups. However, there's a few analyst outliers with credibility who do not believe this feature will appear in the iPhone 4G; I think it's a foregone conclusion.
• iAd, enabling more robust, fuller-page, interactive video ads -- what better to support app development
• Apple iChat included (Mobile iChat)
• Improved battery (Engadget suggests 16% larger over 3GS; 5.25 WHr at 3.7V)
• Faster speed (ARM Cortex-A9 processor); think the A4, but improved
  
• Updated SDK with over 1,000 new APIs, 100 new features
• Unified email inbox
• An easier choice for large businesses, as it is to include improved data protection and larger-scale wireless app delivery for enterprises
• iBooks (similar to app for iPad; think Kindle for iPhone)
• Limited "Game Center" features -- to begin by offering ability to post your scores on a leaderboard to compare them with others; possibly also multiplayer games where the "Center" auto-links up like players of similar skills, intents
• Noise-cancelling mic
• Less than 5 grams heavier than iPhone 3GS
• "Home" button along the side
• Screen lock button along same side

My additional assumptions:

• To be released 7 June 2010 at WWDC.
• Available on Verizon and AT&T in the US.
• Videochat would not be available on 3G/CDMA networks or would only be available over 3G with a supplemental data plan. (Available on WiFi only.)
• Removable back panel for swapping batteries -- available in EU countries only.

What? Still not enough? Fine. Here's a little bit of smartphone porn for the iPhone 4G.