I don't remember setting it, but it's set to 'never' already. I'll keep an eye on whether it really disconnects as often as I thought.
For reference as to how to set it: a quick google says: Settings>Wireless and Network>WIFI Settings>Menu Button>Advanced>Sleep Policy to Never .

Reading comments, the smartphone + data plan + commercial SIP sounds the most promising. Sometimes I like to Figure It All Out, but right now asterisk and all sound like a bit too much trouble. I did buy the Nexus S, and am (as expected) finding mobile internet/maps/e-mail tremedously useful (or at least fun). And, I barely talk at all –I've had a PAYG for the last two or three years –so having that on the side (and forwarding missed calls to it) might be a good backup for occasional loss of coverage.

The people in the Clear store said they didn't throttle per-service (I specifically asked about SIP/VoIP, and they also mentioned torrents), though I don't know if they're authoratative (or entirely knowledgeable; they misheard 'SIP phone' as ZipPhone at first). But yes, a detail that's important to check.

On the Nexus S, you can set Airplane Mode but (possibly with the help of an app) still turn WiFi on, which leaves me with a "no service" message on the unlocking screen, but avoids any later complaints. Perhaps that's something similar to be done on the E71?

That sounds like it might be a great option. I'm totally new to SIP. Any recommendations for SIP providers, or what to look for? (Of course, I can go do my homework, but...)

Cost per month with iPhone (AT&T):
$40 for 450 minutes
$25 + $10/GB over for 2GB
$10 + 10/ea for 1000 text messages
Plus residential internet:
~$40

Cost per month with Ideal Plan:
$40 for unlimited data

Good to hear it has potential to work. My concern so far is mostly the missing incoming calls: I often turn on the phone (just testing in the house) and see that the WiFi has disconnected, despite apparent strong signal. (However, Line2's pricing seems more straightforward than Skype's, though Skype is what I initially looked at.)

Another reason to move to Canada, then.

Informative indeed! I glanced at Virgin Mobile but glossed over them mostly; but that plan sounds appealing. I do already have the Nexus S, but it's still within the 30-day return window (and although I enjoy it I probably wouldn't be heartbroken to switch); however, it looks like it might talk to their network.

Any comments on Virgin Mobile's speed / restrictions on tethering?

Actually, I'm trying to have both, and have them be the same thing. When I'm at home, the Spot (or other mobile ISP) would be my home internet connection; when I'm mobile, it would be mobile broadband, as well as voice (over WiFi to my phone). So it's very informative (if sad) to hear that VoIP over mobile is jittery – which matches some, but not all, of the few tests I've done.

I'm trying out a Clear Spot on a relative, and with the range set to 'high' (in probably very good coverage) it lasted about 4 hours driving around (so set to low probably it'd do better). It's good to hear corroborative reports. Most of the time, too, I anticipate being either at home or at work, so a quarter-day mobile time might be okay. If not, I'd rather buy and carry an extra battery than buy voice+data+sms+home. (However, it's not clear that the Spot warns you in any useful way of a low battery –I haven't tested that yet.)

I've been testing Skype and Line2 calls over WiFi in a house with Clear, and latence doesn't seem to be an issue. The lag is noticeable if I'm within earshot of the person whose cell phone I'm calling, but without that second channel for comparison it's not significant. There are however some issues with calls dropping (the Nexus S's ability to hold onto a WiFi signal isn't impressing me so far; it gets 1/4 when a laptop in the same location gets 4/4 for signal strength); and the Google Voice + pbxes.org + Sipdroid solution has pretty bad quality.

I tried this (and was very excited to find it), but sipdroid seems to have trouble staying logged in to pbxes, so I only sometimes get incoming calls (or can make outgoing calls); and call quality is ok-to-poor (but it is free).

Thank you! Scientific rigor and grammatical rigor seem to be decoupled for the OP.