If 7.1 discrete audio means anything to you, you'll need and HDMI connection on the receiver. If the up-and-coming audio codecs aren't really of any concern to you, then I'd say no HDMI isn't necessary. And you can always by a switcher after the fact if something does come up and you require more video connections over HDMI.
The Onkyo TX-SR605 is coming out within a month, and if the 604 was any indication of how it's price will decline over the next year, expect to be able to pick one up for under $300 by next spring (if you're not in a huge rush). If it lives up to its specs, it'll be my next receiver for sure.
HDMI isn't just about digital video. No, seriously! (some of you laughed, I know it)
Your HD cable may have an HDMI output, but that's definitely something that I would plug directly into the TV for video (via HDMI) and use a standard optical audio cable to send sound to the receiver. Why? Because the cable signal can't go beyond a Dolby Digital 5.1 standard, there's no need to waste 1 of the 2 HDMI inputs on the 605. You'd get all the advantages for the video (100% digital and all that stuff), and just-as-good digital audio. The PS3 or a standalone HD player would be something that I would run through the receiver, because HDMI is the only thing that can transfer a 7.1 LPCM or encoded signal to the receiver; an optical cable in this case would compromise some of the new capabilities of the audio available on Blu-Ray/HD-DVD.
Sound for me is important, clearly. But if it's not a big deal for you, then HDMI on a receiver may not be either, saving you some cash... However, if it is important, HDMI may be a necessity, not just a convenience. It depends on how important this stuff is to you.