I'm looking for suggestions on good multi-country/"global" SIM
options. I have an unlocked tri-band GSM phone.

I've reached the stage in my career where I'll start traveling abroad
from my home in the US. My first is later this month, to Taiwan and
Hong Kong. Future trips will include these and other Asian
countries[1], plus various European locations (UK, Germany, France,
Italy, etc.). I expect to take one to five such a year.

These trips will all be in lumps; that is, I'll almost never go just
to Hong Kong, or just to France. Instead, I'll likely do, say, a week
in Hong Kong and a week in Taiwan, or a week with a day each in
London, Paris, Amsterdam, Geneva, and Rome.

My priorities are, in order:
1. One number that I can be reached at anywhere. (Except in North
  America, of course, where I'll keep using my CDMA phone.)
2. No callback-based solution. I don't care how automated the process
  is; if I call someone I want to call right away.
3. Cost. I will get reimbursed for all work-related calls but I still
  don't want to spend any more than I have to.

Priority 1 eliminates buying and keeping a whole bunch of
country-specific SIM cards. (Except one that I'll get to later on.)
Beside the inconvenience I wouldn't want to deal with each one
expiring over time before I make my next trip that that country,
anyway, not to mention trying to refill each card from outside that
country.

So far I've found two solutions:

Mobal (<URL:http://www.mobalrental.com/>
Pros: No upfront charges or subscriptions. No expiration date. All
calls billed to credit card on a pay-as-I-go basis.
Cons: More expensive than Talk Abroad, even without the fact that
Mobal doesn't offer any free incoming anywhere.

Talk Abroad (<URL:http://www.cellularabroad.com/>)
Pros: Cheaper rates than Mobal in most places I've seen. Free incoming
in most of Europe plus Taiwan.
Cons: SIM card expires nine months after last usage. Prepaid credit
expires 12 months after last usage. $59 upfront cost (albeit with a
30-minute credit).

Both services provide UK phone numbers.

So far the Talk Abroad option looks more appealing, even with the
upfront cost. I expect to travel abroad often enough for the credit
expiration to not be an issue, and I can always keep the SIM card from
expiring by sending an SMS.

What else, if anything, am I missing? One alternative would be if
there's some single-country prepaid SIM card provider somewhere whose
card's roaming costs are inexpensive enough so as to make using said
SIM card in other countries economical (I don't mind having a Czech or
Australian or Moldavan phone number if that's the case), but were
there such a provider I imagine I'd have heard about it by now.

[1] Including Japan and Korea, which I realize are special cases which
I won't try to deal with here.